We present the case of a young man with acute monocytic leukemia (French-American-British classification:M5) and systemic hyperfibrinolysis with severe bleeding. Although fibrinolysis is usually mild and secorulary to disseminated intravascular coagulation, its role as a primary and dominant factor in rare cases of leukemia warrants that its presence be sought as a cause of abnormal bleeding. Decreased serum plasminogen and increased serum plasmin determined by synthetic substrate assay and a negative protamine paracoagulation test are crucial findings. Use of high-dose epsilon-aminocaproic acid was effective in treating this complication. A transient increase in fibrinolytic activity coincident with the early effect of antileukemic treatment suggested that plasminogen activator and/or fibrinolytic protease substances were released from leukemic cells. Fibrinolytic activity subsequently disappeared with reduction in the population of leukemic cells
