Proteolytic degradation of fibrin (fibrinolysis) is mediated by plasminogen and its activators, tissue-type plasminogen activator (tPA1) and urokinase (uPA). Fibrinolysis is critical for preventing thrombus growth and restoring blood flow following thrombotic vascular occlusion. Plasminogen activator inhibitor-1 (PAI-1), a member of the serine protease inhibitor (serpin) superfamily, is the principal inhibitor of tPA and uPA in the fibrinolytic system. High levels of circulating PAI-1 are associated with a number of thrombotic diseases. In animal studies, transgenic mice overexpressing human PAI-1 develop spontaneous thrombosis, whereas PAI-1-deficient mice are more resistant to venous or arterial thrombosis. Furthermore, inhibition of PAI-1 activity prevents thrombus formation in animal models. The antithrombotic effects of PAI-1 inhibition are achieved by enhancing endogenous fibrinolytic activity without directly affecting blood coagulation and platelet function. Phenotypic analysis of PAI-1 deficiency in both human and mouse suggests that inhibition of PAI-1 will not lead to severe bleeding or other major adverse effects. Thus, PAI-1 inhibitors represent a new class of antithrombotic drugs with a possible wider therapeutic index than conventional antiplatelet and anticoagulant agents. This review summarizes the role of PAI-1 in thrombotic diseases and recent progress in the development of small molecule PAI-1 inhibitors.