The implementation of leniency programs is considered a success both at a EU Community level and in individual member states. The paper discusses the value of ex officio investigations for cartel detection in light of leniency and complaint-based cases. Are ex officio investigations still needed? Should a competition authority concentrate its scarce resources exclusively on the prosecution of leniency or complaint-based cases or follow a proactive market monitoring policy? It is argued that investigations triggered ex officio are an important complementary enforcement tool to the other passive instruments available to a competition authority. A bottom-up methodology for triggering inspections based on economic criteria is presented allowing for a more proactive cartel policy.