Do You Have to Enforce Your Patent? Duty, Demand Letters, and the Cost of Doing Nothing

Summary. A patent owner has no legal duty to enforce a patent, and no duty to file suit after sending a cease and desist letter — unlike a trademark, a patent does not weaken, narrow, or lapse because infringement is tolerated. Never suing does not affect validity, but it steadily erodes the remedy: the six-year damages window under § 286 closes year by year, injunctive relief becomes harder under eBay, and licensing leverage drops. The genuine danger is the specific pattern of threatening and then going quiet, which supplies both elements of an equitable estoppel defense and can bar the entire claim, past and future, as it did in Aspex Eyewear after three years of silence and Radio Systems v. Lalor after four and a half. Meanwhile the letter itself hands the recipient a declaratory judgment vehicle, an unstarted IPR clock, and exposure under the bad-faith assertion statutes now on the books in 34 states. The practical rule: decide whether to litigate before you send the letter, and if you are not ready to file, preserve the position with a tolling agreement or documented follow-up rather than silence.

A client called me a few years back with a familiar problem. She had a patent on a data-synchronization method, she had found a competitor shipping something that looked a lot like her claims, and eighteen months earlier her prior counsel had sent a stern letter demanding the competitor stop. The competitor's lawyer wrote back denying infringement. Nobody responded. The file went cold.

Now the competitor had grown, raised a round, and was quoting the patented feature in its sales deck. She wanted to know two things. Had she broken some rule by not suing? And had she wrecked her patent by waiting?

Those are the right questions, asked in slightly the wrong order. Here is the short version.

The short answer

You have no legal duty to enforce a patent. Nothing in Title 35 obligates a patent owner to police infringement, and unlike a trademark, a patent does not weaken, dilute, or lapse because you tolerate competitors using it. The Supreme Court settled this in 1908 and Congress reinforced it in the 1988 patent misuse amendments.

You have no legal duty to file suit after sending a cease and desist letter. A demand letter is not a contract with the recipient and not a promise to the court. You can send one and never sue.

But the patent is not the thing at risk. The remedy is. Never suing does not invalidate your patent. It can, however, shrink your damages, cost you an injunction, hand the other side control of the forum, and in the specific pattern my client had stumbled into, wipe out your claim entirely under the doctrine of equitable estoppel.

That last point deserves emphasis, because it inverts the intuition most people bring to this. Staying quiet from the very beginning is legally safer than threatening someone and then going silent. The letter is what creates the expectation. The silence afterward is what defeats you.

The rest of this article works through all three questions in detail, with the cases, the statutes, and a set of practical checklists at the end. I have tried to write it so a software founder can follow it without a patent bar registration number, while still being precise enough for the lawyer who has to act on it.

Part One: Where the "duty to enforce" idea comes from, and why it does not apply to patents

Almost every founder who asks me this question has absorbed the idea from trademark law. That is not their fault. Trademark law genuinely does impose something like a policing obligation, the rule gets repeated constantly in startup circles, and nobody bothers to say which kind of intellectual property they are talking about.

Trademarks really do have a use-it-or-lose-it problem

A trademark exists to identify a single commercial source. That function is empirical. It either works in the minds of consumers or it does not. So trademark law ties the strength of the right to the reality on the ground.

If you let competitors use a confusingly similar mark, three bad things can happen. Your mark's scope of protection narrows, because a crowded field of similar marks means each one gets a thinner zone of exclusivity. Your mark can become generic, the way aspirin, escalator, and thermos did, at which point the registration is cancellable and the right is simply gone. And under 15 U.S.C. § 1127, a mark is deemed abandoned when the owner's conduct causes it to lose significance as an indication of source, which includes uncontrolled or naked licensing.

That is a real duty. Not a duty to sue every infringer, but a duty to police the marketplace well enough that the mark keeps doing its job. If you handle trademark portfolios, this is the reflex you develop, and it is correct.

It is also completely inapplicable to patents.

A patent is a right to exclude, not a duty to exclude

Section 154(a)(1) of the Patent Act describes what a patent gives you: the right to exclude others from making, using, offering to sell, selling, or importing the claimed invention. A right to exclude. Not a mandate to exclude, not a license conditioned on excluding.

The Supreme Court addressed the argument head-on in Continental Paper Bag Co. v. Eastern Paper Bag Co., 210 U.S. 405 (1908). The accused infringer argued that a patentee who neither practiced its patent nor licensed it to anyone had forfeited its claim to equitable relief. The patentee in that case was doing exactly that, sitting on a paper-bag machine patent while continuing to run older equipment. The Court rejected the argument, reasoning that excluding competitors is the essence of the right the patent confers, and that it is the privilege of any property owner to use or not use the property.

Congress ratified this in the patent misuse amendments of 1988. Under 35 U.S.C. § 271(d)(4), a patent owner is not guilty of misuse or illegal extension of the patent right by reason of having refused to license or use any rights to the patent. The provision exists precisely because courts had been flirting with the opposite rule.

Continental Paper Bag is not entirely untouched. When the Supreme Court granted certiorari in eBay Inc. v. MercExchange, L.L.C., 547 U.S. 388 (2006), it directed the parties to brief whether Continental Paper Bag should be reconsidered on the question of when an injunction is appropriate. The resulting opinion changed injunction practice significantly, which matters later in this article. But eBay addressed remedies, not the existence of the underlying right. Non-use still does not forfeit a patent.

There is no such thing as patent abandonment through non-enforcement

Worth saying plainly, because the terminology causes confusion. "Abandonment" appears in patent law in two places, and neither has anything to do with declining to sue infringers.

The first is abandonment of an application during prosecution, which happens when an applicant fails to respond to an Office action within the statutory period. That kills the application, not an issued patent.

The second is abandonment, suppression, or concealment of an invention under the old pre-AIA § 102(g) priority rules, which is now largely of historical interest for patents governed by the first-inventor-to-file system.

Neither doctrine is triggered by watching a competitor infringe for a decade. There is no patent equivalent of genericide, no patent equivalent of naked licensing, and no provision under which tolerated infringement narrows your claim scope. Claim scope is fixed by the claim language, the specification, and the prosecution history. Market conduct after issuance does not touch it.

The only affirmative duty is writing checks to the USPTO

For a utility patent, the one thing you actually have to do to keep the patent alive is pay maintenance fees under 35 U.S.C. § 41(b). Section 41(b)(1) sets them at three years and six months, seven years and six months, and eleven years and six months after grant; § 41(b)(2) gives a six-month grace period, after which the patent expires as of the end of that period, and a surcharge applies under § 41(a)(7). Miss them and the patent expires, at which point reinstatement requires showing the delay was unintentional under § 41(c)(1) and there is no guarantee of success.

Design patents and plant patents carry no maintenance fees at all.

This is a low bar and an easy one to trip over. I have seen more patents die from a docketing failure after a change of counsel or an acquisition than from any substantive defect. If you own patents, the calendar is the compliance program.

Where a duty to enforce can actually exist: read your own paper

The absence of a statutory duty does not mean you have no duty. It means the duty, if any, is one you created yourself. Several categories come up regularly.

Exclusive license agreements. This is the most common source by a wide margin. An exclusive licensee has paid for exclusivity, and exclusivity is worth nothing if third parties infringe freely. Most well-drafted exclusive licenses address this directly, either by obligating the licensor to enforce, by granting the licensee a right to enforce in the licensor's name if the licensor declines within a stated period, or by giving the licensee a royalty offset or termination right if infringement goes unaddressed.

There is a procedural wrinkle underneath the contract question. Under Independent Wireless Telegraph Co. v. Radio Corp. of America, 269 U.S. 459 (1926), an exclusive licensee that lacks all substantial rights in the patent generally cannot sue alone and must join the patent owner, and a licensor who refuses to join can be added involuntarily. Whether your licensee has "all substantial rights" is a fact-intensive question about what the agreement actually transferred, and it is litigated constantly. If you are drafting an exclusive license, decide this deliberately rather than discovering the answer in a standing motion three years later.

FRAND and standards commitments. If you have declared patents essential to a standard and committed to license on fair, reasonable, and non-discriminatory terms through a standard-setting organization, your enforcement freedom is constrained in the opposite direction. The commitment limits your ability to seek injunctive relief and obligates you to license willing implementers. This is its own body of law and beyond the scope of this article, but if you are in that posture, the general "no duty to enforce" rule is not your operative rule.

Joint development and collaboration agreements. These often contain enforcement allocation provisions specifying who has first right to sue, who bears costs, and how recoveries are split. If you are the party with the first right and you decline, check whether declining transfers the right to your counterparty.

Security agreements and financing covenants. Lenders who take patents as collateral sometimes include covenants requiring the borrower to preserve and defend the collateral. Failing to act against a known infringer may be a covenant breach.

Fiduciary duty. A board that lets a company's most valuable asset be appropriated without inquiry has an exposure problem in theory. In practice, the business judgment rule gives directors substantial room, and litigation is an expensive discretionary decision squarely within it. A documented, reasoned decision not to sue is very defensible. An undocumented failure to notice is less so.

The technologist's version of this problem: your own licenses may have already given the patent away

Here is where the software context changes the analysis, and where I see companies get surprised.

If your product incorporates code under Apache License 2.0, section 3 of that license grants every recipient a patent license covering claims necessarily infringed by the contribution, and it includes a defensive termination clause: anyone who initiates patent litigation alleging that the work constitutes patent infringement loses their patent license under the agreement as of the filing date. GPLv3 contains its own patent provisions in section 11. The Mozilla Public License 2.0 has an analogous structure.

The practical consequence is that "can I enforce this patent against that company" is not purely a patent question. It is also a question about what you granted, deliberately or otherwise, when your engineering team contributed to an upstream project, shipped under a permissive license, or joined a patent non-aggression arrangement like the Open Invention Network's Linux System definition.

I have had this conversation more than once: a client identifies a clear infringer, we build the claim chart, and then someone remembers that a core module of the product went out under Apache 2.0 three years ago and the accused party is a downstream user of that module. That is not an enforcement problem you solve with a better demand letter.

Add to this the various public patent pledges, some binding and some aspirational, that companies made during the 2010s and then forgot about. If your company ever published a patent pledge, it is part of the enforcement analysis.

The audit sequence I recommend before any assertion campaign:

1. Inventory every open-source license under which your own products ship, and identify the patent grant and defensive termination language in each.
2. Identify every contribution your employees made to upstream projects under a Contributor License Agreement, and read the patent grant in that CLA.
3. Check for any standards participation, patent pledge, or patent pool membership.
4. Check for any covenant not to sue, settlement release, or license in your own agreement archive that names or could cover the target.

That last one catches people constantly, because releases in old settlement agreements are frequently drafted broadly enough to cover affiliates, successors, and customers.

Part Two: You sent a cease and desist letter. Are you now obligated to sue?

No. Nothing in the Patent Act, the Federal Rules, or general contract principles converts a demand letter into an enforceable promise to litigate. You can send a letter, get a hostile response, and let the matter drop. Patent owners do this all the time, often for good reasons: the claim chart looked weaker after the response, the target turned out to be judgment-proof, budget disappeared, or the business decided the relationship was worth more than the claim.

That said, the letter is not free. It moves several things in the recipient's favor the moment it lands.

The letter hands the recipient the keys to the courthouse

Before 2007, the Federal Circuit applied a "reasonable apprehension of imminent suit" test to declaratory judgment jurisdiction, which let patent owners send fairly aggressive letters without exposing themselves to a DJ action. The Supreme Court dismantled that test in MedImmune, Inc. v. Genentech, Inc., 549 U.S. 118 (2007), replacing it with the general Declaratory Judgment Act standard: whether, under all the circumstances, there is a substantial controversy between parties with adverse legal interests of sufficient immediacy and reality to warrant relief.

The Federal Circuit applied the new standard weeks later in SanDisk Corp. v. STMicroelectronics, Inc., 480 F.3d 1372 (Fed. Cir. 2007), holding that DJ jurisdiction existed where a patentee had communicated a detailed infringement position, even though it had also said it did not intend to sue. A conditional disclaimer does not undo a concrete assertion.

The practical result: once you send a letter that identifies patents, identifies products, and asserts infringement, assume the recipient can file a declaratory judgment action tomorrow.

Personal jurisdiction used to be the patent owner's backstop, on the theory that sending letters into a state was not enough to be haled into court there. The Federal Circuit has steadily eroded that. In Jack Henry & Associates v. Plano Encryption Technologies, 910 F.3d 1199 (Fed. Cir. 2018), the court rejected a per se rule that enforcement letters can never support jurisdiction. In Trimble Inc. v. PerDiemCo LLC, 997 F.3d 1147 (Fed. Cir. 2021), it found jurisdiction proper based on roughly twenty-two licensing communications over a three-month period, stating there is no general rule that demand letters can never create specific personal jurisdiction. And in Apple Inc. v. Zipit Wireless, Inc., 30 F.4th 1368 (Fed. Cir. 2022), the court reversed a dismissal that had read its precedent as establishing exactly such a bright-line rule.

Read those three together and the pattern is clear. Volume, specificity, in-person meetings, claim charts, draft complaints, and short deadlines all push toward jurisdiction where the recipient sits. A single measured letter is weaker ground for the DJ plaintiff, but "weaker" is not "safe."

What you lose when the recipient wins the race to the courthouse is substantial. You lose forum selection, which in patent litigation is worth real money. You lose the framing of the case, because a complaint for declaratory judgment of invalidity and non-infringement sets a different tone than a complaint for infringement. You may lose the ability to fix problems in your infringement theory before it is locked into pleadings. And you become a defendant in a case about your own asset.

The letter starts an inter partes review clock that does not run in your favor

This one catches patent owners who think of the demand letter as a low-commitment opening move.

Under 35 U.S.C. § 315(b), an inter partes review petition is time-barred if filed more than one year after the petitioner is served with a complaint alleging infringement of the patent. Service of a complaint. Not receipt of a demand letter.

So a demand letter gives the recipient every incentive to evaluate an IPR, and it starts no clock at all against them. They can sit on that option indefinitely while you sit on yours. If you eventually sue, they get a fresh year from service. If you never sue, they can file whenever a business reason arises, including years later when they want to clear the space.

The asymmetry is worth internalizing. The letter transfers optionality to the other side.

Baseless threats create their own liability

A patent owner who threatens litigation without a reasonable basis is exposed on several fronts, and this exposure exists whether or not suit is eventually filed.

Fee-shifting. Under 35 U.S.C. § 285, a court may award reasonable attorney fees to the prevailing party in exceptional cases. Octane Fitness, LLC v. ICON Health & Fitness, Inc., 572 U.S. 545 (2014), defined an exceptional case as one that stands out from others with respect to the substantive strength of a party's litigating position or the unreasonable manner in which the case was litigated, and lowered the standard of proof to a preponderance. Pre-suit conduct, including the adequacy of the pre-filing investigation and the character of demand letters, factors into that analysis.

State-law tort claims. Assertions of patent infringement to third parties, including a target's customers, can support claims for tortious interference, trade libel, or unfair competition. Federal patent law preempts those claims unless the patentee acted in bad faith. Zenith Electronics Corp. v. Exzec, Inc., 182 F.3d 1340 (Fed. Cir. 1999); Globetrotter Software, Inc. v. Elan Computer Group, Inc., 362 F.3d 1367 (Fed. Cir. 2004). Bad faith is a high bar, incorporating First Amendment and Noerr-Pennington concerns, but it is not unreachable, and the analysis has both objective and subjective components.

Antitrust. Enforcement of a patent obtained by knowing and willful fraud on the Patent Office can support a Sherman Act § 2 claim under Walker Process Equipment, Inc. v. Food Machinery & Chemical Corp., 382 U.S. 172 (1965). Separately, bad-faith enforcement of a patent known to be invalid or not infringed can support antitrust liability under the Handgards line, subject to the sham litigation standard of Professional Real Estate Investors, Inc. v. Columbia Pictures Industries, Inc., 508 U.S. 49 (1993).

State bad-faith assertion statutes. Since Vermont enacted the first one in 2013, more than thirty states have passed statutes targeting bad-faith patent assertion, most of them aimed specifically at demand letters. Counts published by practitioner sources vary between roughly thirty and thirty-five depending on how broadly the category is drawn, so confirm your own jurisdiction rather than relying on a headline number.

These statutes matter more than patent lawyers sometimes assume, and one feature is directly relevant to the question this article is about. Virginia's statute, at Va. Code § 59.1-215.2(B)(6), lists among the factors evidencing bad faith that "[t]he assertion of patent infringement is deceptive, or the person threatens legal action that cannot legally be taken or that is not intended to be taken." Enforcement is reserved to the Attorney General under Va. Code § 59.1-215.3. Read that clause carefully. A state legislature has written into law the proposition that threatening a suit you do not intend to file is evidence of bad faith. Other state statutes contain comparable language.

Other common factors across these statutes: failure to include the patent number and the identity of the owner, failure to conduct a claim-to-product analysis before sending, demanding a response in an unreasonably short period, and offering a license at a price untethered to any reasonable valuation.

The enforcement mechanisms vary. Some statutes create a private right of action; some, like Virginia's, reserve enforcement to the attorney general. Some provide for bonds. In Micron Technology, Inc. v. Longhorn IP LLC, No. 2023-2007 (Fed. Cir. Dec. 18, 2025), the Federal Circuit dismissed an interlocutory appeal from an eight million dollar bond that an Idaho district court had ordered under the Idaho Bad Faith Assertions of Patent Infringement Act, finding it lacked appellate jurisdiction over the bond order. The preemption question the case teed up was left for another day, but the practical lesson landed: a state statute can impose a very expensive obstacle early in a case, and appellate relief may not be available until final judgment.

Federal Trade Commission attention. The FTC brought an enforcement action against MPHJ Technology Investments over a mass demand-letter campaign — its first use of consumer protection authority against a patent assertion entity — settling in November 2014 and approving the final consent order on March 17, 2015. In re MPHJ Technology Investments, LLC, FTC File No. 142-3003. The order bars misrepresenting that a patent has been widely licensed and misrepresenting that suit will be filed or is imminent. Volume campaigns aimed at small businesses draw regulatory attention in a way that individual disputes between competitors do not.

The ethics dimension for the lawyer signing the letter

Separate from the client's exposure, counsel has independent obligations. Model Rule 4.1 prohibits knowingly making a false statement of material fact or law to a third person. Model Rule 8.4(c) prohibits conduct involving dishonesty, fraud, deceit, or misrepresentation. Rule 11 obligations do not attach to a letter, but they attach to the complaint you may later file, and the pre-filing investigation for that complaint should have been substantially complete before the letter went out.

"We will file suit within fourteen days" is a representation. If nobody has authorized suit, nobody has retained litigation counsel, and no budget exists, that representation is at best careless. I draft around it. "Our client is prepared to pursue all available remedies" says the same thing to a sophisticated reader without asserting a fact about intent that may not be true.

Part Three: Does never suing hurt the patent?

The patent, no. The claim, potentially yes, and in one scenario, fatally.

Validity does not care what you did after issuance

A patent's validity is determined under 35 U.S.C. §§ 101, 102, 103, and 112. Eligible subject matter, novelty, non-obviousness, written description, enablement, and definiteness. Every one of those is assessed against the state of the art as of the relevant filing date and against the four corners of the patent document.

Nothing in that analysis includes an input for how vigorously the owner enforced. An unenforced patent and an aggressively enforced patent with identical claims and identical prosecution histories have identical validity. There is no doctrine by which sitting still makes claims narrower or the specification weaker.

So if the question is "will my patent be worth less as a legal instrument if I never sue," the answer is no.

If the question is "will I be able to recover what I should have recovered," the answer gets complicated.

The damages window closes six years at a time

Section 286 provides that no recovery shall be had for any infringement committed more than six years prior to the filing of the complaint. This is not a statute of limitations in the ordinary sense. It does not bar the claim. It is a limitation on the damages period, measured backward from filing.

The arithmetic is simple and unforgiving. Every year you wait is a year of damages you can never recover, permanently, no matter how strong the case turns out to be. If the infringer has been selling ten million dollars a year of accused product and you wait four years to file, you have not delayed four years of recovery. You have destroyed it.

For a growing infringer this cuts in a counterintuitive direction. The early years of infringement are usually the low-revenue years, so the six-year window may not be where the money is. But for a mature infringer at steady state, four years of waiting is four years of gone.

Marking, notice, and what the letter is actually good for

Section 287(a) is the reason a demand letter has value even if you never sue.

If you make or sell a product covered by the patent and you fail to mark it with the patent number, you cannot recover damages for any period before the infringer received actual notice of the infringement. Constructive notice through marking, or actual notice through a communication that charges infringement with specificity, is the trigger.

The Federal Circuit's decisions in the Arctic Cat Sales v. Bombardier Recreational Products line address both halves of this. 876 F.3d 1350 (Fed. Cir. 2017) sets the burden framework, placing an initial burden of production on the accused infringer to identify unmarked products before the patentee must prove compliance. 950 F.3d 860 (Fed. Cir. 2020) holds that a patentee's cessation of sales of unmarked products does not cure the prior failure to mark. The practical significance is that for an unmarked patentee, the demand letter is the event that starts the damages clock running at all.

So a letter followed by silence is not entirely wasted. It fixes a notice date. But that notice date does you no good if a different doctrine bars the claim, which brings us to the one that actually matters.

Equitable estoppel: the doctrine that can end the case outright

This is the answer to my client's real question, and it is the reason the "threaten and go quiet" pattern is the single worst enforcement posture available.

The framework comes from A.C. Aukerman Co. v. R.L. Chaides Construction Co., 960 F.2d 1020 (Fed. Cir. 1992) (en banc). Three elements, all of which the accused infringer must prove:

1. The patentee, through misleading conduct or silence, led the alleged infringer to reasonably infer that the patentee did not intend to enforce its patent against that infringer.
2. The alleged infringer relied on that conduct.
3. The alleged infringer would be materially prejudiced if the patentee were now allowed to proceed.

If the elements are met, the court weighs the totality of the equities. And the consequence is not a reduction in damages. It is a complete bar to all relief, past and prospective. A successful estoppel defense functions as something close to a royalty-free license for the accused product going forward. The Federal Circuit has described the result in terms of an implied license in cases like Wang Laboratories, Inc. v. Mitsubishi Electronics America, Inc., 103 F.3d 1571 (Fed. Cir. 1997).

Why silence alone is usually not enough. Under Aukerman, silence by itself will not create estoppel unless there was a clear duty to speak or the continued silence reinforces the defendant's inference of acquiescence. Hemstreet v. Computer Entry Systems Corp., 972 F.2d 1290 (Fed. Cir. 1992), put it as a requirement that mere silence be accompanied by some other factor indicating the silence was sufficiently misleading as to amount to bad faith.

A patent owner who has never said anything to anyone has usually not engaged in misleading conduct. There is no communication to be misled by.

Why a demand letter supplies the missing element. The letter is the "something more." It establishes that the patentee knew about the accused product, formed a view that it infringed, and communicated an intention to act. When nothing follows, the recipient can reasonably read the silence as a withdrawal of the charge.

Aspex Eyewear, Inc. v. Clariti Eyewear, Inc., 605 F.3d 1305 (Fed. Cir. 2010), is the cleanest illustration. Aspex sent letters in 2003 identifying several patents, stating its understanding that Clariti's products might be covered and asserting a strong intention to fully and vigorously enforce its rights in what it called a very urgent and serious matter. Clariti responded denying infringement of the patents Aspex had focused on and asking for specifics. Aspex went silent as to the patent later asserted. Three years passed while Clariti kept selling the accused eyewear. When Aspex sued in 2007, the district court dismissed on equitable estoppel and the Federal Circuit affirmed.

Three years. From a patentee that had used the phrase "fully and vigorously enforce."

Radio Systems Corp. v. Lalor, 709 F.3d 1124 (Fed. Cir. 2013), reached the same result on a longer timeline. Bumper Boy sent a demand letter to Innotek in February 2005 accusing an electronic dog collar of infringement, then said nothing for roughly four and a half years while Innotek expanded its product line and was acquired by Radio Systems. The Federal Circuit affirmed estoppel as to the patent identified in the letter. It reversed as to a related continuation-in-part patent that had not issued until after the letter, on the ground that claims which have not issued cannot be asserted and therefore cannot be the subject of misleading silence.

That second holding matters, and it drove the outcome in a case people often cite for the opposite proposition. In John Bean Technologies Corp. v. Morris & Associates, Inc., 887 F.3d 1322 (Fed. Cir. 2018), John Bean had told Morris's customers in 2002 that Morris's poultry chillers infringed. Morris wrote back challenging validity and demanding a response. John Bean said nothing for about eleven years, then sought ex parte reexamination, amended its claims substantially, obtained a reexamination certificate in 2014, and sued six weeks later. The district court granted summary judgment on laches and equitable estoppel. The Federal Circuit reversed both. Laches fell because SCA Hygiene came down while the appeal was pending. Estoppel fell on the narrow ground that the asserted reexamined claims did not exist in their present form at the time of the 2002 correspondence and had been substantively altered since, so John Bean could not have engaged in misleading silence with respect to those specific claims.

Do not read John Bean as a safe harbor for eleven years of silence. Read it as a claim-identity rule. The estoppel defense attaches to claims that existed and were assertable when the misleading conduct occurred. Substantively amend the claims and you may reset the analysis, though as the same parties learned on the second appeal, 988 F.3d 1334 (Fed. Cir. 2021), you then walk into equitable intervening rights under 35 U.S.C. § 252, applied to reexamined claims through § 307(b), which cost John Bean the case anyway.

The line that keeps ordinary licensing conversations safe. Not every assertion followed by quiet creates estoppel. Meyers v. Asics Corp., 974 F.2d 1304 (Fed. Cir. 1992), holds that a suggestion of infringement coupled with an offer to license, followed by silence, does not by itself support estoppel. Licensing negotiations in which a patentee asserts infringement do not automatically create estoppel either.

The distinguishing features tend to be tone, specificity, and deadline. A letter that says "we believe there may be overlap between your product and our portfolio and would like to discuss licensing" reads as an opening offer. A letter that says "your Model X infringes claims 1, 7, and 12 of the '482 patent, cease immediately, respond within ten days or we will file suit" reads as a charge. Silence after the first is ambiguous. Silence after the second is a withdrawal.

Reliance and prejudice are real elements, not formalities. High Point SARL v. Sprint Nextel Corp., 817 F.3d 1325 (Fed. Cir. 2016), and the district court decisions applying it show that defendants lose estoppel arguments regularly by failing to prove they actually relied on the patentee's silence, or by failing to show material prejudice. Prejudice comes in two forms: evidentiary, meaning lost witnesses, discarded documents, and faded memories; and economic, meaning capital investments, product launches, hiring, or acquisitions the defendant would not have undertaken had it believed suit was coming.

Note how the reliance element interacts with corporate transactions. In Radio Systems, the acquisition of the original letter recipient was part of the prejudice story. If your target gets acquired during your period of silence, the acquirer's diligence file will contain the letter and the absence of any follow-up, and that absence becomes evidence.

Laches is mostly gone, and what replaced it

For years, laches was the standard delay defense in patent cases. SCA Hygiene Products Aktiebolag v. First Quality Baby Products, LLC, 580 U.S. 328 (2017), ended that for damages claims, holding that laches cannot bar recovery for infringement occurring within the six-year window of § 286. The Court followed the reasoning of Petrella v. Metro-Goldwyn-Mayer, Inc., 572 U.S. 663 (2014), which had reached the same conclusion for copyright: where Congress has legislated a limitations period, courts should not superimpose a judge-made timeliness rule on top of it.

SCA Hygiene expressly preserved equitable estoppel, describing it as a defense long recognized as available in actions at law against patentees who behave unscrupulously. Which is why the doctrine that used to be the second-string delay defense is now the whole game.

Whether laches survives as a defense to equitable relief, particularly a permanent injunction, remains less settled after SCA Hygiene. In practice the same delay evidence gets deployed through the eBay factors instead, which reaches a similar place by a different route.

Prosecution laches: a different delay problem, recently narrowed

Prosecution laches concerns delay in the Patent Office rather than delay in enforcement, but it belongs in any discussion of the cost of waiting, and it moved significantly in 2025.

The doctrine renders a patent unenforceable when it issued only after unreasonable and unexplained delay in prosecution amounting to an egregious misuse of the statutory patent system, plus prejudice attributable to the delay. Cancer Research Technology Ltd. v. Barr Laboratories, Inc., 625 F.3d 724 (Fed. Cir. 2010). The Federal Circuit applied it in Hyatt v. Hirshfeld, 998 F.3d 1347 (Fed. Cir. 2021), and Personalized Media Communications, LLC v. Apple Inc., 57 F.4th 1346 (Fed. Cir. 2023).

In 2023, Judge Alsup used the doctrine to hold Sonos's "Zone Scene" patents unenforceable against Google, finding that Sonos had waited thirteen years, from a 2006 provisional filing until 2019, to claim the feature at issue. That decision alarmed anyone running a continuation strategy, which is most software patent portfolios.

The Federal Circuit reversed in Google LLC v. Sonos, Inc., No. 2024-1097 (Fed. Cir. Aug. 28, 2025), a nonprecedential opinion that reinstated a jury verdict of roughly $32.5 million. The court did not reach the "unreasonable delay" question. It resolved the case on prejudice, holding that Google had presented no evidence to support its assertion that its investment in the accused products began in 2015, and that the district court had therefore abused its discretion.

The practical takeaway for portfolio owners is that prosecution laches survives but requires proof of specific, causal prejudice, and that prejudice is hard to establish where the invention was publicly disclosed in a published parent application before the accused infringer began its own development. Continuation practice is not, standing alone, an egregious misuse of the system.

Injunctions get harder every year you wait

Under eBay, a permanent injunction requires the traditional four-factor showing: irreparable injury, inadequacy of legal remedies, a balance of hardships favoring the plaintiff, and no disservice to the public interest.

Years of tolerated infringement damage the first two factors directly. A patentee who watched a competitor sell the accused product for five years has a hard time explaining why the sixth year causes irreparable harm, and an even harder time explaining why money is suddenly inadequate when money was apparently acceptable for five years. Courts say this out loud.

The same delay evidence hurts preliminary injunctions worse, where the movant must show likelihood of success and irreparable harm on a compressed record and delay is treated as strong evidence against urgency.

Damages theory and licensing leverage both erode

The reasonable royalty analysis under 35 U.S.C. § 284 runs through the Georgia-Pacific factors, from Georgia-Pacific Corp. v. U.S. Plywood Corp., 318 F. Supp. 1116 (S.D.N.Y. 1970). Several of those factors interact badly with a history of non-enforcement.

Factor 1 looks at royalties the patentee has received for licensing the patent. If you have licensed cheaply to settle nuisance disputes, that becomes your ceiling. Factor 4 examines the patentee's established policy of maintaining its monopoly by not licensing others, which is worth something only if the policy is actually maintained. Factor 5 considers the commercial relationship between the parties. Factor 12 looks at customary profit or royalty in the industry, and widespread unlicensed use in the market is evidence about what the industry actually pays, which is nothing.

Outside litigation, the effect on negotiating leverage is more immediate. Sophisticated counterparties run the same analysis I have described here. A patent that has been asserted against nobody, in a market with visible unlicensed practice of the claims, is a patent whose owner is presumed unwilling or unable to enforce. That shows up as a discount in every license negotiation, every diligence process, and every valuation.

Willfulness cuts in a direction people do not expect

Under Halo Electronics, Inc. v. Pulse Electronics, Inc., 579 U.S. 93 (2016), enhanced damages under § 284 are available for egregious infringement behavior, at the court's discretion and without the rigid two-part test the Federal Circuit had previously applied.

Your demand letter is excellent evidence of the infringer's knowledge, which is a predicate for willfulness. That is a point in your favor and a reason the letter has enduring value.

But the discretion runs both ways. The Read factors that courts consult in deciding whether and how much to enhance include the patentee's own conduct. A patentee who charged infringement, went silent for four years, and then reappeared asking for treble damages is making an argument about the defendant's egregiousness from a position that invites scrutiny of its own.

Evidence decays, and the burden of that decay is yours

The unglamorous version of the delay problem. Custodians leave. Laptops get wiped on the standard retention schedule. Source control history gets migrated and truncated. The engineer who can explain why the accused implementation works the way it does takes a job in another country. Marketing materials from the relevant period disappear when the website is redesigned.

You bear the burden of proving infringement and damages. Degraded evidence hurts the party with the burden. And the same decay supplies the evidentiary prejudice element of the estoppel defense you are now facing.

If you are contemplating enforcement at any horizon, a litigation hold on your own materials should issue early, and third-party evidence about the accused product, including archived versions of documentation, product pages, and downloadable binaries, should be preserved contemporaneously rather than reconstructed later.

Summary table: what delay actually costs

Part Four: Why "threaten, then vanish" is the worst available posture

Put the three parts together and an asymmetry appears that is worth stating explicitly, because it is the opposite of what most business owners assume.

Never saying anything is legally safe as to estoppel. There is no misleading conduct, so the first Aukerman element usually fails. You still lose damages to the § 286 window and to § 287 notice problems, and you still weaken your injunction case, but the claim survives.

Saying something and following through is obviously fine.

Saying something and then going quiet is the pattern that produces Aspex and Radio Systems. You have created the expectation of enforcement, then defeated it. You have handed the recipient a document proving you knew about their product and thought it infringed, which is exactly the evidence they need for the first element, and then you have supplied the silence that satisfies it.

You have also, in that same period, given them a declaratory judgment vehicle they can deploy at a moment of their choosing, an unstarted IPR clock, and in thirty-four states, at least a colorable argument that threatening a suit you did not intend to file is evidence of bad faith.

This is why I tell clients that the decision to send a demand letter and the decision to litigate should be made at the same time, by the same people, with the budget question answered. Not because the letter commits you to sue. It does not. But because the letter starts a clock that only you can stop, and stopping it requires doing something, not nothing.

How to keep a claim alive without filing

If you have sent a letter and are not ready to sue, there are ways to preserve the position. None of them is "wait and see."

Tolling and standstill agreements. A written agreement that the parties will not assert delay-based defenses for a defined period, in exchange for the patentee not filing during that period, converts a dangerous silence into a documented negotiation. The infringer often wants this too, because they would rather not be sued this quarter. This is the cleanest solution available and it is underused.

Documented, periodic contact. A short letter every few months stating that the patentee's position is unchanged, that it continues to evaluate its options, and that it reserves all rights keeps the file from reading as abandoned. The Aspex problem was three years of nothing, not three years of unhurried correspondence. Weigh this against the Trimble problem: volume and specificity of contacts push toward personal jurisdiction in the recipient's home forum. There is a middle band between "silence" and "twenty-two communications in three months," and that band is where you want to be.

Reframe as licensing. Meyers teaches that a suggestion of infringement plus a license offer, followed by silence, is materially different from a charge followed by silence. If the commercial goal was always a license, say so and negotiate like it. Keep the correspondence oriented toward business terms rather than escalating threats.

Explicit deferral with a stated reason. "We are deferring further action pending the outcome of the co-pending reexamination of the '482 patent and will contact you when it concludes" gives the recipient a reason for the silence that undercuts any inference of abandonment. It also has the advantage of being true when it is true.

File and stay. In some situations the right answer is to file the complaint to stop the § 286 clock and the estoppel accrual, then immediately seek a stay for mediation or pending a parallel proceeding. Expensive, but it converts an eroding position into a fixed one.

Covenant not to sue, if you are truly done. If the answer is that you will never enforce against this party, a covenant not to sue moots declaratory judgment jurisdiction. Super Sack Manufacturing Corp. v. Chase Packaging Corp., 57 F.3d 1054 (Fed. Cir. 1995); Already, LLC v. Nike, Inc., 568 U.S. 85 (2013). The covenant has to be broad enough to cover future conduct, not just past sales, to accomplish that. Revolution Eyewear, Inc. v. Aspex Eyewear, Inc., 556 F.3d 1294 (Fed. Cir. 2009). This is a real concession and should be treated as a decision, not an exit.

Practical takeaways

Before you send a cease and desist letter

1. Complete the claim chart first. Element by element, claim by claim, against the actual accused product or a reasonable reverse-engineering of it. If you cannot chart it, you cannot charge it, and several state statutes make the absence of a pre-suit comparison an explicit bad-faith factor.
2. Run your own invalidity analysis. Assume the recipient will file an IPR. Ask what you would file if you were them.
3. Audit your own license grants. Open-source patent clauses, contributor agreements, patent pledges, standards commitments, and old settlement releases. Do this before the letter, not after the response.
4. Decide the litigation question now. Who would file, in what venue, on what budget, on what timeline. Not because you are committing, but because you need the answer before you make a representation about it.
5. Assume you will be sued in the recipient's home district. Choose your language and your volume of contact accordingly, and be sure you can live with that forum.
6. Confirm the marking position. If you sell a covered product and have not been marking, the letter is your notice date under § 287. Get the specificity right so it counts.
7. Check the state statute where the recipient sits. Include the patent number and the identity of the real party in interest, give a reasonable response period, and make any license demand defensible as a valuation.
8. Do not write a deadline you have no intention of honoring.
9. Preserve your own evidence and capture the accused product's public footprint as it exists today.
10. Calendar the follow-up before you send. Ninety days is a reasonable default.

If you already sent a letter and let it go cold

1. Determine how long the silence has run and against which specific claims. Timeline the correspondence precisely.
2. Assess whether the recipient changed position in reliance. New products, new facilities, headcount, financing, acquisition. That is the prejudice element and it is where the case will be decided.
3. Check whether any asserted claim has been substantively amended since the correspondence, through reexamination or reissue. That can reset the estoppel analysis, at the cost of walking into intervening rights under § 252.
4. Do not restart with a fresh threat before you have decided whether to sue. A second silence is much worse than the first.
5. Consider whether a tolling agreement is achievable. If the other side has any doubt about its own position, it may prefer certainty to a lawsuit.
6. Recalculate the damages exposure using the § 286 window from a realistic filing date, not from the date of first infringement. The number is often much smaller than the client expects, and that changes the business decision.

If you received a letter and the patentee went quiet

1. Preserve the correspondence and every internal document reflecting decisions you made afterward. Reliance is an element you have to prove, and contemporaneous evidence is worth far more than testimony later.
2. Document investment, launches, hiring, and expansion during the silent period, with dates. That is your prejudice showing, and the Google v. Sonos reversal is a reminder that assertions without evidentiary support fail.
3. Watch for continuations in the patent family. Under Radio Systems, estoppel does not attach to claims that had not issued when the letter was sent.
4. Evaluate the IPR option deliberately. Your § 315(b) clock does not start until you are served with a complaint, so you hold the timing.
5. If you want certainty rather than a defense, a declaratory judgment action in your home district is available, and MedImmune plus Trimble and Zipit make it more available than it was fifteen years ago.

A note on how I would have advised my client differently

Looking back at the file, the mistake was not the letter. The letter was fine. The mistake was that nobody wrote down what would happen if the response was a denial, and nobody owned the ninety-day follow-up. The claim did not die from a considered decision. It eroded from an unowned calendar entry.

That is the actual failure mode in most of these situations. Not aggression, not timidity, but the absence of a decision one way or the other. Patent law will let you enforce or not enforce as you choose. It is considerably less forgiving about the space in between.

I handle patent, trademark, and technology disputes for software companies and other technology businesses, including pre-suit assessment and enforcement strategy. If you are holding a patent you have not decided what to do with, or a letter you sent and never followed up on, that conversation is usually worth having before another year runs off the clock.

[CUSTOMIZE: insert firm's standard blog disclaimer here. Suggested language, subject to your bar's advertising rules: "This article is for general informational purposes only, is not legal advice, and does not create an attorney-client relationship. Patent enforcement decisions depend heavily on the specific facts of your situation and the law may have changed since publication. Consult a licensed attorney about your particular circumstances."]

Current as of August 2026.

Primary authority

• 35 U.S.C. § 154(a)(1) — the grant is a right to exclude, which is the textual root of there being no duty to enforce.
• 35 U.S.C. § 271(d)(4) — refusing to license or use the patent is not misuse.
• 35 U.S.C. § 286 — the six-year look-back on damages, the provision that makes delay expensive without making it fatal.
• 35 U.S.C. § 287(a) — marking and actual notice; for an unmarked patentee the demand letter is what starts damages running.
• 35 U.S.C. § 284 and § 285 — damages and exceptional-case fees.
• 35 U.S.C. § 41(b) and § 41(c)(1) — the only affirmative maintenance obligation, and the unintentional-delay route back.
• 35 U.S.C. § 315(b) — the IPR bar runs from service of a complaint, not from a demand letter.
• 35 U.S.C. § 252 and § 307(b) — absolute and equitable intervening rights after reissue or reexamination.
• 15 U.S.C. § 1127 — trademark abandonment, the contrast that generates the whole misconception.
• Continental Paper Bag Co. v. Eastern Paper Bag Co., 210 U.S. 405 (1908) — no duty to use or license.
• A.C. Aukerman Co. v. R.L. Chaides Construction Co., 960 F.2d 1020 (Fed. Cir. 1992) (en banc) — the equitable estoppel elements and the "clear duty to speak," abrogated as to laches by SCA Hygiene.
• SCA Hygiene Products Aktiebolag v. First Quality Baby Products, LLC, 580 U.S. 328 (2017) — laches is no defense to damages within § 286; estoppel survives.
• Aspex Eyewear, Inc. v. Clariti Eyewear, Inc., 605 F.3d 1305 (Fed. Cir. 2010) and Radio Systems Corp. v. Lalor, 709 F.3d 1124 (Fed. Cir. 2013) — threaten-then-vanish, three years and four-plus years respectively.
• MedImmune, Inc. v. Genentech, Inc., 549 U.S. 118 (2007) and SanDisk Corp. v. STMicroelectronics, Inc., 480 F.3d 1372 (Fed. Cir. 2007) — the declaratory judgment exposure a letter creates.
• eBay Inc. v. MercExchange, L.L.C., 547 U.S. 388 (2006) — why every year of tolerated infringement makes an injunction harder.
• Arctic Cat Inc. v. Bombardier Recreational Products Inc., 876 F.3d 1350 (Fed. Cir. 2017) and 950 F.3d 860 (Fed. Cir. 2020) — the marking burden framework, and the rule that stopping unmarked sales does not cure the earlier failure.
• Google LLC v. Sonos, Inc., No. 2024-1097 (Fed. Cir. Aug. 28, 2025) (nonprecedential) — prosecution laches reversed for want of prejudice.
• Micron Technology, Inc. v. Longhorn IP LLC, No. 2023-2007 (Fed. Cir. Dec. 18, 2025) — interlocutory appeal from an $8 million Idaho bad-faith bond dismissed for want of jurisdiction; preemption left open.
• Va. Code § 59.1-215.2 — a representative bad-faith assertion statute, including the factor on threats "not intended to be taken."

Related articles

• Patent Litigation Toolkit: A Roadmap and Research Guide — the operational sequence if you decide to file.
• Comprehensive Guide to Patent Infringement Litigation: From Summary Judgment Denial to Post-Trial — what "being ready to sue" actually costs.
• Patent Post-Grant Proceedings: IPR, PGR, and Reexamination at the PTAB — the § 315(b) clock your letter does not start.
• Patent Marking Compliance: A Practical Checklist — the cheapest way to stop losing damages.
• How to License Your Patent: From Valuation to Term Sheet — the leverage that erodes while you wait.
• Patent Ownership, Assignments, and Standing: Chain-of-Title Problems That Sink Cases — confirm you can sue before you threaten to.
• Trademark Cease and Desist Letters: Sending and Responding — the regime patents are constantly confused with.
• Maintaining Trademark Registrations — the genuine use-it-or-lose-it obligation.
• Open Source Software: Licenses, Compliance, and Risk — the defensive-termination clauses that may have already licensed your adversary.
• Drafting a Demand Letter: A Practical Checklist — before you send anything.
• Patent Basics: A Plain English Guide — the fundamentals underneath all of this.

This article is provided for general informational purposes and does not constitute legal advice. Patent enforcement strategy is fact-specific, equitable estoppel turns on the particular sequence of communications and the accused infringer's reliance, and the state bad-faith assertion statutes differ materially in scope, remedies, and who may enforce them. Recent decisions discussed here — including the Federal Circuit's nonprecedential disposition in Google LLC v. Sonos, Inc. and its jurisdictional dismissal in Micron Technology, Inc. v. Longhorn IP LLC — may be subject to further proceedings. Consult qualified patent counsel before sending a demand letter, before letting one go unanswered, and before deciding that doing nothing is free.