Hemorrhagic fever with renal syndrome in Korea (Korean hemorrhagic fever) is an acute viral disease characterized by fever, hemorrhage and renal failure. In Korean patients, the disease manifests more distinctive bleeding tendencies than those of hemorrhagic fever with renal syndrome found in western countries. To investigate the nature and role of the coagulation, fibrinolysis, kinin and immune system in the pathogenesis of such a hemorrhagic manifestation, alterations of these systems were assessed from the early phase of the disease. Decreased platelet count and shortened platelet survival were observed with giant platelets in the peripheral blood. The marked prolongations of bleeding time, prothrombin time and partial thromboplastin time were noticed with the decreased plasma activities of coagulation factors II, V, VIII, IX and X. Shortened half life of fibrinogen, increased fibrinogen-fibrin degradation product, with decreased plasma levels and activities of plasminogen, alpha 2-plasmin inhibitor and antithrombin III were found. On thrombelastogram, the existence of procoagulant activity was confirmed, and prolonged reaction time and clot formation time with decreased maximum amplitude were observed. The appearance of circulating immune complexes was found along with decreased C3 and normal C4 in the serum. Significant decrease of serum C3 was evident in the patients with disseminated intravascular coagulation. These findings of coagulopathy were normalized within ten days of the illness in most cases. Therefore, it can be concluded that disseminated intravascular coagulation and thrombocytopenia in the early phase, and azotemia developing later might play an important role in the pathogenesis of bleeding tendency in Korean hemorrhagic fever.