## Facts

- Festo owned two patents covering an improved magnetic rodless cylinder used in conveying systems.
- During prosecution of one patent, Festo amended claims after a § 112 rejection to add limitations requiring (i) a pair of one-way sealing rings and (ii) a magnetizable outer sleeve.
- During reexamination of the second patent, Festo added a limitation specifying sealing rings.
- SMC manufactured a competing cylinder using a single two-way sealing ring and a nonmagnetizable sleeve.
- Festo sued SMC for infringement under the doctrine of equivalents.
- The district court found for Festo, rejecting SMC’s prosecution-history-estoppel defense.
- After remand in light of Warner-Jenkinson, the Federal Circuit (en banc) adopted a “complete bar,” holding that any narrowing amendment made to satisfy the Patent Act creates estoppel that eliminates all equivalents for the amended element.
- The Supreme Court granted certiorari to review the Federal Circuit’s complete-bar rule.

## Issues

1. Whether prosecution history estoppel can arise from narrowing amendments made to satisfy any Patent Act requirement (including § 112), or only from amendments made to avoid prior art.
2. Whether, once estoppel applies, it categorically bars all reliance on the doctrine of equivalents for the amended claim element.
3. What standard governs the scope of subject matter surrendered by a narrowing amendment and the patentee’s ability to prove that a particular equivalent was not surrendered.

## Decision

- The Court held that prosecution history estoppel may arise from any narrowing amendment made for a substantial reason related to patentability, not only prior-art-based amendments.
- The Court rejected the Federal Circuit’s complete-bar approach.
- A narrowing amendment creates a presumption that the patentee surrendered all equivalents between the original and amended claim scope for the amended element.
- The presumption is rebuttable; the patentee may show the particular accused equivalent was not surrendered.
- The Court vacated the Federal Circuit’s judgment and remanded for application of the rebuttable-presumption framework to the accused equivalents.

## Legal Principles

- The doctrine of equivalents may treat an accused product as infringing despite nonliteral differences when the substituted feature is insubstantial.
- Prosecution history estoppel limits equivalents when the patentee narrows a claim to obtain a patent; the amendment signals surrender of some subject matter.
- Estoppel is triggered by a narrowing amendment made for a substantial reason related to patentability, regardless of which statutory requirement prompted it.
- When the prosecution record does not make the amendment’s reason clear, the patentee bears the burden to show it was not made for a patentability-related reason; otherwise estoppel applies.
- A narrowing amendment creates a rebuttable presumption of surrender of all equivalents for the amended element, covering the territory between the original and amended claim.
- The presumption may be rebutted by showing, for the equivalent at issue, that:
  - the equivalent was unforeseeable at the time of the amendment,
  - the amendment’s rationale was only tangentially related to the equivalent, or
  - other circumstances show the patentee could not reasonably have been expected to describe the substitute.

## Conclusion

The Court held that narrowing amendments made for patentability-related reasons trigger prosecution history estoppel, but estoppel does not impose a categorical bar on all equivalents; instead, it creates a rebuttable presumption of surrender that the patentee may overcome for specific equivalents under defined criteria.


Festo Corp. v. Shoketsu Kinzoku Kogyo Kabushiki Co., 535 U.S. 722 (2002)