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.