Warner-Jenkinson Co., Inc. v. Hilton Davis Chemical Co.

Summary

Justice Thomas delivered the Court’s opinion preserving the doctrine of equivalents while requiring an objective, element-by-element infringement inquiry that does not eliminate any claim element. The Court held that prosecution history estoppel depends on the reason for a required claim amendment; the patentee must establish that reason, and an unexplained amendment is presumed to rest on a substantial reason related to patentability. Intent is irrelevant, and equivalence is assessed at the time of infringement rather than patent issuance. The Court declined to prescribe a single verbal test for equivalence or decide the allocation of equivalence determinations between judge and jury. It reversed the Federal Circuit’s judgment and remanded for application of these requirements. No separate opinion appears in the supplied text.