Blood coagulation, fibrinolytic and unspecific proteolytic parameters were investigated in 34 patients with acute myeloid leukemia. An increased activity of the coagulation system, documented by elevated thrombin-antithrombin III-complex (TAT) plasma levels, was found in 91% of the patients; 50% had increased elastase plasma levels. Hyperfibrinolysis, as shown by elevated fibrin split-product D-Dimer plasma levels, was detected in 91% of AML patients. Activation of these enzyme systems was not associated with relevant defects in blood coagulation or fibrinolysis in the majority of the patients investigated. In selected cases of promyelocytic M3 and monoblastic M5 leukemia, however, hypofibrinogenemia and alpha 2-plasmininhibitor deficiency was found, most likely due to depletion of these proteins in the course of disseminated intravascular coagulation and secondary hyperfibrinolysis. Significant correlations were calculated between TAT and fibrinogen (r = -0.57, P less than 0.005), TAT and D-Dimer (r = 0.89, P less than 0.0005), and D-Dimer and alpha 2-plasmininhibitor (r = -0.77, P less than 0.0005) levels. Indications of a pathogenetic importance of primary hyperfibrinolysis or unspecific proteolysis for hypofibrinogenemia and alpha 2-PI deficiency were not found.