We formulate a simple model of optimal defensive disclosure by a dominant firm facing uncertain antitrust enforcement and test its implications using unique data on defensive disclosures and patents by IBM. Our results indicate that stronger antitrust enforcement leads to more defensive disclosure and that defensive disclosure served as an alternative, but less successful, mechanism to patenting at IBM in appropriating returns from R&D. We extend our analysis to two other exceptionally large firms with defensive-disclosure activity, AT&T and Xerox, and show that their patenting propensity declined under increased antitrust enforcement relative to other firms in the industry. We further extend the case study of Xerox and using a differences-in-differences approach we show that patent propensity declined and patent scope increased in patent classes affected by the 1975 consent decree. Overall, we show how these firms used defensive disclosure as a strategy to balance the benefits of patenting with the costs of uncertain antitrust enforcement