We performed a blood coagulation study during the course of disseminated intravascular coagulation (DIC) in 34 patients with hematological malignancies. Risk factors of DIC such as increasing tumor mass, anti-tumor therapy and severe infections were frequently observed at onset of DIC, and influenced the prognosis of DIC. Before the onset of DIC, the DIC score and FDP value were slightly elevated, and they were significantly increased after the onset of DIC. Before the onset of DIC, the level of fibrinogen was significantly increased but it was decreased after the onset of DIC. These hemostatic abnormalities continued for about 2 weeks. Patients with DIC showing prolonged APTT and PT had a poor prognosis. The abnormalities of PT, FDP, fibrinogen, DIC score, FDP-D-dimer and fibrinopeptide A were significantly greater in DIC than in Pre-DIC defined as the period one week before the onset of DIC. FDP-D-dimer was also higher in Pre-DIC patients than in those without DIC. Although protein S and C 4 b binding protein were not decreased in DIC or Pre-DIC, Protein C activity decreased during the course of DIC, suggesting that FDP-D-dimer and Protein C activity were useful for diagnosis of Pre-DIC and DIC.