Rethinking Prosecution History Estoppel

32 Pages Posted: 8 Oct 2003

See all articles by Douglas Lichtman

Douglas Lichtman

University of California, Los Angeles (UCLA) - School of Law


Under the rule of prosecution history estoppel, patent applicants who amend their claims during the course of patent prosecution assume a significant risk: namely, the risk that a court will later construe the changes as concessions that should be read to limit patent scope. This risk is exacerbated by strong evidentiary presumptions under which courts are to assume, unless the patentee presents sufficient evidence otherwise, that every change triggers estoppel, and that the resulting estoppel forfeits everything except that which the revised language literally describes. The justification for these presumptions is that, implemented in this fashion, prosecution history estoppel makes patent scope more predictable. In this Article, I argue that the benefit comes at too high a price. Drawing on a large empirical study of patent prosecution, I show that, because of these evidentiary presumptions, estoppel is dangerously sensitive to differences between patent examiners and differences across technology categories. That is, estoppel treats similar applications in dissimilar ways, not because of differences on the merits, but instead because of the personal characteristics of the examiners involved and because of differences inherent to the types of technology at issue. A better rule, I argue, would minimize the significance of examiner and technology disparities by reversing the current evidentiary presumptions and thus recognizing estoppel only where there is clear evidence that the applicant and the examiner intended to forfeit a given scope of coverage.

Keywords: patent, patent prosecution, patent office, estoppel, prosecution history estoppel, equivalents, doctrine of equivalents, patent examiner, consistency, examiner consistency

Suggested Citation

Lichtman, Douglas Gary, Rethinking Prosecution History Estoppel. U Chicago Law & Economics, Olin Working Paper No. 200; University of Chicago Law Review, Vol. 71, 2004. Available at SSRN: or

Douglas Gary Lichtman (Contact Author)

University of California, Los Angeles (UCLA) - School of Law ( email )

385 Charles E. Young Dr. East
Room 1242
Los Angeles, CA 90095-1476
United States
310-267-4617 (Phone)

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