The equitable defense of prosecution laches
Prosecution laches is an equitable defense that renders a patent unenforceable when the applicant unreasonably and inexcusably delayed the prosecution of a patent application, and that delay materially prejudiced the accused infringer or the public. The defense is rooted in the equitable doctrine of laches — the legal principle that a party who sits on its rights to another's detriment may lose those rights. In the patent context, an applicant who keeps an application pending for years or decades while a market develops around the technology they claim to have invented may be barred from enforcing the eventually-issued claims. The Federal Circuit confirmed prosecution laches is available in Symbol Technologies v. Lemelson Medical (Fed. Cir. 2004) and has since addressed it in Hyatt v. Hirshfeld (Fed. Cir. 2021), among other cases.
Submarine patents: the historical context
Before the AIA's publication requirement (18-month publication of pending applications), patent applications were secret until issuance. Combined with the ability to file continuation applications indefinitely and a patent term measured from issuance (not filing), this created the 'submarine patent' — a patent that stays hidden during prosecution while an industry develops around the claimed technology, then 'surfaces' when issued with claims engineered to cover the developed industry. Jerome Lemelson was the most notorious practitioner of submarine patent strategy, with some of his applications prosecuted for 30–39 years before issuance. When his patents were asserted against major manufacturers, courts used prosecution laches to deny enforcement. The AIA addressed submarine patents directly: applications published 18 months after filing, and patent term is now 20 years from the application's effective filing date (not from issuance), removing the incentive for extended prosecution.
Elements of prosecution laches
To establish prosecution laches, the accused infringer must prove two elements by a preponderance of the evidence: (1) Unreasonable and unexplained delay: the delay in prosecution must be unreasonable under all the circumstances — it is not enough that prosecution was long; there must be no adequate justification (interfering cases, complex prosecution, technical issues). Courts look at the overall prosecution timeline and the applicant's actions or inaction during the delay period; and (2) Material prejudice: the delay must have caused material prejudice to the accused infringer or the public — either reliance prejudice (investments made during the delay period in reliance on the technology being in the public domain) or evidentiary prejudice (loss of evidence relevant to the accused infringer's defenses). Both elements must be proven; showing long delay alone is insufficient.
Legitimate justifications for long prosecution
Not all extended prosecution constitutes unreasonable delay. Patent owners can rebut prosecution laches by showing the delay was justified. Recognized justifications include: interference proceedings (now replaced by derivation proceedings under AIA) — the application was involved in a priority dispute that extended prosecution; appeal proceedings — USPTO rejections were appealed and the appeal process itself caused delay; technical complexity — the nature of the invention required extended prosecution to develop claims; continued invention and development — the applicant was actively engaged in R&D of the claimed technology throughout; government secrecy orders — classified inventions are processed under different timelines. Courts conduct a fact-specific inquiry. An applicant who demonstrated active prosecution activity throughout (filing responses, arguing claims, conducting interviews) is more likely to rebut prosecution laches than one who allowed the application to go abandoned-and-revived multiple times over decades.
Effect of prosecution laches on the patent
If prosecution laches is established, the court renders the patent's claims unenforceable — not invalid. The distinction matters: an invalid patent (lack of novelty, obviousness, etc.) was never a valid patent from the start. An unenforceable patent was valid but cannot be enforced against a specific party who establishes the unenforceability defense, or in some cases, against any party given the public-interest dimension of prosecution laches. Some courts have held prosecution laches to be a defense against all infringement claims (past, present, and future) arising from the same acts of reliance. Others have limited prosecution laches to the specific period of delay-caused prejudice. Because prosecution laches is an equitable doctrine, courts have flexibility in fashioning the remedy — complete unenforceability is most common, but partial relief (barring damages for the delay period) is theoretically available.
Frequently Asked Questions
What is prosecution laches in patent law?
Prosecution laches is an equitable defense to patent infringement based on the patent applicant's unreasonable and inexcusable delay in prosecuting a patent application at the USPTO. When a patent eventually issues after an unusually long prosecution, accused infringers who relied on the public domain during the delay period may assert prosecution laches to render the claims unenforceable. Unlike other patent defenses (invalidity, non-infringement), prosecution laches does not attack the validity of the claims — it asserts that the patent owner forfeited the right to enforce them due to inequitable delay. The Federal Circuit confirmed prosecution laches as a viable defense in Symbol Technologies, Inc. v. Lemelson Medical (Fed. Cir. 2004), reversing a district court decision that had held the defense unavailable. The court held that prosecution laches can render claims unenforceable 'when there has been an unreasonable and unexplained delay in prosecution, and the delay has caused material prejudice to the accused infringer or the public.'
What delay is long enough to support a prosecution laches defense?
There is no bright-line time period that automatically triggers prosecution laches, but courts look at the totality of the prosecution history and whether the delay was unreasonable and unexplained given the circumstances. Historical context: before the AIA (which limited continuation practice for applications filed on or after March 16, 2013), it was possible to maintain patent applications in prosecution for decades through continuation filings — the so-called 'submarine patent' phenomenon. Jerome Lemelson's patents, which formed the basis of the Symbol Technologies case, were prosecuted over 18–39 years. Courts have found that delays of more than 6–8 years without adequate explanation can support prosecution laches claims when combined with prejudice. However, delays of even 10+ years do not automatically establish prosecution laches — the applicant may have legitimate explanations (interference proceedings, complex prosecution issues, appeals, technical complexity of the invention). The AIA effectively eliminated most submarine patent situations by requiring publication of applications 18 months after filing and limiting continuation term.
What is material prejudice and how must it be shown?
Material prejudice is one of the two required elements of prosecution laches (along with unreasonable delay). The accused infringer or the public must have suffered material prejudice as a result of the delay. Types of material prejudice: (1) Reliance prejudice: the accused infringer made substantial investments in designing, manufacturing, or selling the accused product during the delay period, in good-faith reliance on the claimed invention not being patented or otherwise available for use; (2) Evidentiary prejudice: the delay caused the loss of evidence that would have been relevant to invalidity, unenforceability, or license defenses — witnesses have died, documents have been destroyed, and memories have faded; (3) Economic prejudice: an entire industry was established during the delay period in reliance on the technology being in the public domain. Courts require both a causal link between the delay and the prejudice (the prejudice would not have occurred if the patent had issued earlier) and that the prejudice is 'material' rather than de minimis.
How does prosecution laches differ from inequitable conduct?
Prosecution laches and inequitable conduct are both equitable defenses that render patent claims unenforceable, but they are distinct doctrines. Inequitable conduct requires showing: (1) an individual associated with the application made a specific material misrepresentation or omission to the USPTO; and (2) that individual acted with specific intent to deceive the USPTO. The standard is exacting after Therasense v. Becton Dickinson (Fed. Cir. 2011, en banc). Prosecution laches, by contrast, does not require any deception — it is based solely on unreasonable delay and resulting prejudice. A prosecution laches defense can succeed even when the applicant acted in complete good faith throughout the long prosecution. Both defenses, if successful, render all claims of the patent unenforceable (not merely invalid), meaning the claims cannot be asserted in any lawsuit, including those that predate the unenforceability finding. The doctrines can be pled together — a long prosecution might involve both strategic delay (prosecution laches) and specific misrepresentations (inequitable conduct).
Does prosecution laches still matter after the AIA eliminated most submarine patents?
Prosecution laches remains relevant even post-AIA, though its practical importance has diminished for new applications. Reasons why it still matters: (1) Pre-AIA application families: any application that was filed before March 16, 2013 and remains pending can still be subject to the old continuation rules that allowed long prosecutions; if claims are still issuing from these old families, prosecution laches is a live defense; (2) Lemelson-era patents: some Lemelson and similar pre-AIA patents with extremely long prosecution histories are still being asserted; prosecution laches was successfully used against them; (3) Modern continuation delays: even under the AIA, an applicant can maintain continuation applications for many years without publishing them; prosecution laches can apply if the applicant strategically delayed; (4) Foreign counterparts: prosecution laches applies specifically to US patents, but long US prosecution sometimes has foreign equivalents that raise similar issues under other countries' laws. Hyatt v. Hirshfeld (Fed. Cir. 2021) confirmed that prosecution laches remains viable for very long pre-AIA prosecution histories, affirming that Gilbert Hyatt's patents (prosecuted for 25+ years) could be subject to prosecution laches analysis.