Antitrust law has long been mindful of the danger that firms may misuse their patents to facilitate price fixing. Courts and commentators addressing this danger have assumed that patent-facilitated price fixing occurs in a single market. In this Article, we extend conventional analysis to address firms’ patent misuse to facilitate price fixing across multiple products lines. By doing so, we expose gaps in existing agency enforcement and scholarly proposals for reform. Important legal tests that make sense in the single market setting do not carry over to the context we call serial collusion, where certain offenders engage in repeat collusion across product lines. This Article argues that there is an urgent need to recast these tests to address serial collusion of the sort that prevails in the chemicals, auto parts and electronics industries. To support this argument, we develop empirical evidence consistent with the possibility that serial colluders in the chemical industry acquired and used patents to support their collusion, either directly to coordinate and monitor output and pricing or indirectly to deter new firm entry by erecting patent thickets as a barrier to entry. Throughout this Article, we describe the flaws of current antitrust doctrine when it comes to assessing patents and price fixing, suggest doctrinal improvements, and provide guidance to antitrust enforcers about how to better understand and combat serial collusion facilitated by patents.