What is interference estoppel in patent law?

This page is an FAQ based on guidance from the Manual of Patent Examining Procedure. It is provided as guidance, with links to the ground truth sources. This is information only: it is not legal advice.

Interference estoppel in patent law refers to the principle that prevents a party from re-litigating an issue they lost in a previous interference proceeding. As stated in MPEP 2308.03: “If a party loses on an issue, it may not re-litigate the issue before the examiner or in a subsequent Board proceeding.” There are two main types of interference estoppel:

• Estoppel on the merits: A losing party is barred from seeking a claim that would have been anticipated or rendered obvious by the subject matter of the lost count.
• Procedural estoppel: A losing party is barred from seeking relief from the examiner that could have been—but was not—sought in the interference.