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Message: Filing suit after patent expiration?

This has been already addressed in legal forum advice.
Question:

If someone is made aware that a device, which infringes on their patent, is being produced and sold, is there a period of time, within which they must launch legal action, to ensure they retain their rights for remedy.

If they knowingly allow the infringing party to continue to produce and sell the offending product, do they waive their right to prosecute after a period of time?
Answer :
There is no statute of limitations for patent infringement -- as long as the patent is valid when the lawsuit is filed, that is! However, you can only go back 6 years for damages. See 35 U.S.C. 286. So, you could have someone infringing for, say, 15 years, and sue them after 15 years, and there would be no bar on the suit due to statute of lmitations -- but you would only be able to base your damages on the last 6 years of infringement.( You can bring an infringement lawsuit based on an expired patent, so long as you are still within the 6-year damages window of 35 USC 286. You can only sue for damages, of course, and not an injunction, but you can still bring a lawsuit).


That said, there is an equitable common-law principle called "laches" -- basically, what laches are is when it wouldn't be "fair" or "equitable" to allow the suit to go forward. Usually laches comes up in a situation like you described -- where the patent owner knew of the infringement, and let it continue happening in order to build up damages or whatever. In such a case, the patent owner knows the infringement is happening, and could assert his or her rights, but for whatever reason chooses not to -- in that case, where the patent holder, by his own actions, delays the filing of a suit, a court may find that the suit is now "unfair" to the alleged infringer, and may bar the suit under laches.

Laches are fundamentally different from statutes of limitations. They are based on "equity," or "fairness," not on law, and so whether or not laches will be applied will depend on the exact facts of the case. A statute of limitations, on the other hand, is a legal bar to filing suit, and usually easier to assert if the bar has passed.
There may be a presumption that the defense of equitable laches applies if the plaintiff brings suit after the six-year damages period set forth in 35 U.S.C. s 286. In such a case, the burden shifts to the plaintiff to show its delay is not prejudicial and unreasonable vis a vis the defendant-infringer.
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