Festo Corp. v. Shoketsu Kinzoku Kogyo Kabushiki Co.
The Supreme Court ruled that when a patent holder narrows a claim during the application process for any reason related to patentability -- not just to avoid prior inventions -- it can still trigger a legal bar called prosecution history estoppel, which limits claims that a competitor's device infringes by being merely 'equivalent' to the patented invention.
But the Court rejected the lower appeals court's stricter rule that such a bar always blocks every possible equivalent. Instead, it adopted a middle-ground presumption that patent holders can rebut by showing the narrowing amendment could not reasonably have covered the alleged equivalent, sending the case back for further review.
“There is no reason why a narrowing amendment should be deemed to relinquish equivalents unforeseeable at the time of the amendment and beyond a fair interpretation of what was surrendered.”
Explaining why a complete bar on equivalents after a narrowing amendment goes too far.
How it got here: A federal trial court and a Federal Circuit panel ruled for Festo; the full Federal Circuit then reheard the case and ruled for SMC, prompting Festo to seek Supreme Court review.
The Case in Depth
What happened
Festo owns two patents on a magnetic rodless cylinder used in industrial machinery. During the patent application process, Festo amended its claims to add sealing rings and specify a magnetizable sleeve material. A competitor, SMC, later sold a similar device using a different sealing ring design and a nonmagnetizable sleeve, and Festo sued claiming the device was an equivalent that infringed its patents.
The question before the Court
If an inventor narrows a patent claim to satisfy any patent-law requirement, does that always block them from later claiming a competitor's device is an equivalent?
Why it matters
The ruling affects how far patent protection extends whenever an inventor amends a claim during the application process -- a nearly universal occurrence. Businesses and inventors can still pursue infringement claims against close copies of their inventions even after amending claims, but they now bear the burden of showing a particular competing product wasn't given up during that amendment.
What changes now
The case returns to the lower courts to apply the new presumption-based standard. Festo will have the chance to show that its narrowing amendments did not surrender the specific features SMC's device uses -- the sealing ring design and sleeve material -- to prove infringement under the doctrine of equivalents. This is a final merits ruling on the legal standard, though the ultimate outcome for Festo and SMC remains undecided.
What this does not decide
The Court did not decide whether SMC's device actually infringes Festo's patents. It only settled the legal standard for when prosecution history estoppel applies and how far it reaches, leaving the case-specific application of that standard to further proceedings in the lower courts.
How the Court got there
The legal reasoning, step by step
- The Court explained that prosecution history estoppel prevents an inventor who narrowed a patent claim during the application process from later claiming that the narrowed-away territory was merely an equivalent covered by the doctrine of equivalents (the rule letting patents cover close copies, not just literal matches).
- The Court held that this estoppel can arise from any narrowing amendment made for a reason tied to the Patent Act's requirements, not only amendments made to avoid pre-existing inventions (prior art), because what matters is whether the amendment narrowed the claim's scope, not the specific reason for narrowing it.
- The Court then rejected the Federal Circuit's 'complete bar' rule, which had held that once estoppel applies, the inventor loses all rights to claim any equivalent for that narrowed element, reasoning that language is always an imperfect description of an invention and a narrowing amendment does not show the inventor foresaw every possible equivalent.
- Instead, the Court adopted a presumption: a narrowing amendment presumptively surrenders the territory between the old and new claim language, but the inventor can rebut that presumption by showing the equivalent was unforeseeable at the time, only tangentially related to the amendment's purpose, or otherwise not something the inventor could reasonably have been expected to describe.
- Applying this framework, the Court found Festo had not yet rebutted the presumption for its sealing-ring and sleeve-material amendments, since both features were expressly addressed in the prosecution history, leaving the question open for further proceedings.
Doctrinal impact
Cases affected by this decision
Reaffirms Warner-Jenkinson Co. v. Hilton Davis Chemical Co. (520 U. S. 17)
The Court relied on this earlier ruling as still-good law establishing that estoppel and equivalents doctrine remain settled and flexible.
Reaffirms Graver Tank & Mfg. Co. v. Linde Air Products Co. (339 U. S. 605)
The Court cited this case as continuing authority protecting patent holders from insubstantial copying.