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Non-Mutual Offensive Collateral Estoppel in Private Antitrust Litigation: Lessons from the Microsoft Cases

72 Pages Posted: 11 Oct 2012  

Michael E. Jacobs

Fiscalía Nacional Económica

Date Written: October 10, 2012


For nearly 100 years, antitrust policy in the United States has reflected the important role of private enforcement by allowing private litigants to benefit from successful government enforcement actions. With the enactment of section 5(a) of the Clayton Act in 1914, prior judgments obtained by the government were given prima facie effect in follow-on litigation by private parties. Following the U.S. Supreme Court’s 1976 decision in Parklane Hosiery Co. v. Shore, non-mutual offensive collateral estoppel has since supplanted section 5(a) as the tool of choice in many private antitrust lawsuits. That doctrine, however, has been described as “detailed, difficult, and potentially dangerous,” and the cautious approach that some courts have adopted in the antitrust context is in tension with the policy preference expressed in the Clayton Act that favors giving private plaintiffs the full benefits of prior government judgments.

The Microsoft antitrust litigation presents one of the most interesting and challenging examples of that tension, and thus provides important lessons for future cases. This article seeks to provide a framework, based on lessons learned in Microsoft, for the application of non-mutual offensive collateral estoppel in antitrust cases that gives effect to the policy considerations underlying section 5(a) of the Clayton Act, while remaining mindful of the concerns identified by the U.S. Supreme Court in Parklane Hosiery. It argues that a broader application of the doctrine is warranted in the antitrust context and that, in order to give private plaintiffs the full benefit of prior government enforcement actions, all issues decided in the earlier proceeding that were “necessary” to each analytical step undertaken by the court in deciding, under the applicable substantive standard, that the antitrust laws had been violated should be given preclusive effect.

Keywords: antitrust, collateral estoppel

Suggested Citation

Jacobs, Michael E., Non-Mutual Offensive Collateral Estoppel in Private Antitrust Litigation: Lessons from the Microsoft Cases (October 10, 2012). Available at SSRN: or

Michael E. Jacobs (Contact Author)

Fiscalía Nacional Económica ( email )


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