We study the impact of antitrust enforcement on insiders’ trading profits. The notion is that trading profits and product market collusion are related because collusion enhances insiders’ informational advantage. Using staggered crosscountry adoption of leniency laws, we show stronger enforcement curbs trading profits. Likewise, trading profits increase when enforcement decreases due to nearby DOJ office closures. Sensitivity of trading profits to antitrust enforcement varies with a firm’s likely role in collusive arrangements, its monitoring effectiveness, degree of information asymmetry between insiders and outsiders, and the industry’s propensity for collusion. We also document that stronger antitrust enforcement lowers the informativeness of insider trades.