The blood coagulation and fibrinolysis disorders are common complications observed in patients with acute leukemias, particularly in acute myelogenous leukemia. These abnormalities are mediated by thromboplastic substances released from the blast cells and the alteration of hemostatic properties of vascular endothelium. The plasma concentration of D-dimer (cross-linked fibrin degradation products), measured by enzyme immunoassay, using monoclonal antibodies, serves as a specific marker of the coagulation activation and fibrinolysis system. In our study, the plasma concentration of D-dimer was investigated in 142 patients with acute leukemia during clinical course--at the time of initial diagnosis, complete remission, relapse or in cases resistant to chemotherapy. It has been revealed that, at the time of initial diagnosis, the plasma level of D-dimer was elevated in most patients, irrespective of the type of acute leukemia. However, the initially elevated plasma concentration of D-dimer was significantly lower when complete remission had been achieved. Furthermore, in the majority of cases of relapse or resistance to chemotherapy, a further increase of plasma concentration of D-dimer is commonly observed.