Festo Corp. v. Shoketsu Kinzoku Kogyo Kabushiki Co. is the Supreme Court decision that calibrated the boundary between the doctrine of equivalents and prosecution history estoppel. Festo held that a narrowing claim amendment during prosecution presumptively surrenders the territory between the original and amended claim — but the presumption is rebuttable, with three escape paths a patentee can take. The decision is the operative law on every patent infringement case that turns on equivalents rather than literal infringement.
Decided May 28, 2002, 535 U.S. 722.
Background
The doctrine of equivalents lets a patentee win infringement against a product that doesn't literally meet every claim limitation but is "equivalent" to the claimed invention — typically meaning it performs substantially the same function in substantially the same way to achieve substantially the same result. The doctrine guards against accused infringers who make trivial substitutions to escape literal claim language.
Prosecution history estoppel is the corresponding limit. When a patentee amends a claim during prosecution to overcome prior art (or to satisfy any other patentability requirement), the patentee gives up the surrendered claim territory — and can't reclaim it later under the doctrine of equivalents.
Festo Corporation owned U.S. Patent Nos. 4,354,125 and 3,779,401, both covering magnetically coupled rodless cylinders (a kind of pneumatic actuator). During prosecution, both claims were narrowed: a "pair of resilient sealing rings" was added to the '125 claim, and a "magnetizable" sleeve replaced an "aluminum" sleeve in the '401 claim. Both amendments overcame prior-art rejections.
Shoketsu's competing devices used a single sealing ring and a non-magnetizable sleeve — they didn't literally infringe. Festo argued for infringement under the doctrine of equivalents.
The Federal Circuit, sitting en banc, held that Festo had completely surrendered any equivalent — applying an absolute bar to the doctrine of equivalents whenever a narrowing amendment was made for any reason related to patentability.
Festo appealed.
The holding
The Supreme Court unanimously reversed. Justice Kennedy's opinion rejected the absolute bar in favor of a rebuttable presumption of estoppel:
The patentee, as the author of the claim language, may be expected to draft claims encompassing readily known equivalents. A patentee's decision to narrow his claims through amendment may be presumed to be a general disclaimer of the territory between the original claim and the amended claim. There are some cases, however, where the amendment cannot reasonably be viewed as surrendering a particular equivalent.
The Court identified three rebuttal paths:
- Unforeseeability. The equivalent may have been unforeseeable at the time of the amendment.
- Tangential rationale. The reason for the amendment bears no more than a tangential relation to the equivalent in question.
- Some other reason. "Some other reason" exists suggesting that the patentee could not reasonably have been expected to describe the equivalent.
The patentee carries the burden of showing one of these applies.
What Festo did and didn't do
What Festo did:
- Replaced the Federal Circuit's absolute bar with a rebuttable presumption.
- Identified three concrete rebuttal categories.
- Left a narrow window for the doctrine of equivalents to survive a narrowing amendment.
What Festo didn't do:
- Make rebuttal easy. The presumption is, in practice, hard to overcome. Courts apply Festo skeptically.
- Eliminate the importance of careful claim drafting. The fastest way to preserve doctrine-of-equivalents scope is to avoid narrowing amendments in the first place — and the second-fastest is to put a clear rationale for any amendment on the record.
Aftermath
Post-Festo, doctrine-of-equivalents arguments navigate a structured framework:
- Was there an amendment that narrowed the claim? If no narrowing amendment, Festo doesn't apply — the patentee runs the standard equivalents analysis.
- Was the amendment for a reason related to patentability? Almost always yes. Most amendments are made to overcome prior art under § 102 or § 103.
- What is the surrendered territory? Generally, everything between the original and amended claim language.
- Can the patentee rebut? Has it shown unforeseeability, tangentiality, or "other reason"? Each requires record evidence — usually from the prosecution history itself.
The Federal Circuit has applied Festo hundreds of times. The pattern: defendants win when a narrowing amendment maps cleanly onto the accused product's exact differences. Patentees win when they can identify a tangential rationale or genuinely unforeseeable equivalent.
What it means for defendants
For a defendant arguing non-infringement, Festo is a powerful tool whenever the patentee invokes the doctrine of equivalents:
- Map the prosecution history. Identify every claim amendment. Look for amendments that narrowed the claim along the same dimension where you differ from the literal claim.
- Match the amendment to the equivalent. If the amendment narrowed "from X to Y" and your product is "X without Y," prosecution history estoppel almost certainly bars the equivalent.
- Force the structured analysis. Don't let the patentee skip to function-way-result without first running through Festo's presumption. The presumption applies first; the equivalents analysis only reaches the merits if the presumption is rebutted.
For doctrine-of-equivalents fights, the prosecution history is your single most important record. Pull it from PAIR or PEDS, read every Office action and response carefully, and build the Festo argument before discussing function-way-result.
For literal-infringement fights — which are most fights — Festo doesn't reach. There the focus is claim construction under Markman and prior-art invalidity.
Bottom line
Festo is the gravitational law of the doctrine of equivalents. Any narrowing amendment during prosecution carries presumptive estoppel against the equivalent of the surrendered scope; the presumption is rebuttable but hard to overcome. The defense workflow: literal infringement fails on claim construction, then doctrine of equivalents fails on Festo.
Read the opinion: Festo Corp. v. Shoketsu Kinzoku Kogyo Kabushiki Co., 535 U.S. 722 (2002).
This article is for general education and is not legal advice.