Antitrust authorities search public documents to discover anticompetitive mergers. Thus, investor disclosures may alert them to deals that would otherwise escape scrutiny, creating disincentives for managers to divulge transactions. We study this behavior in publicly traded US companies. First, we estimate a regression discontinuity that exploits mandatory disclosure thresholds stipulated by securities law. We find that releasing information to investors poses antitrust risk. Second, we present a method for measuring undisclosed merger activity that relies on financial accounting reporting requirements. We find that undisclosed mergers total $2.3 trillion between 2002 and 2016.