Two patients with acute promyelocytic leukemia and severe bleeding associated with hypofibrinogenemia were studied. The markedly shortened whole blood clot lysis time and dilute clot lysis time suggested that the defect was an increase in fibrinolysis. Although disseminated intravascular coagulation could not be totally excluded as an alternative mechanism, excessive fibrinolysis was confirmed as the pathogenic cause by the prompt response to the administration of tranexamic acid. The low circulating plasminogen, alpha 2 plasmin inhibitor level and the presence of alpha 2 plasmin inhibitor-protease complex in both patients suggested that the increased fibrinolysis probably resulted from the liberation of plasminogen activator from the promyelocyte.