The pathophysiology of bleeding manifestations in hemorrhagic fever with renal syndrome (HFRS) was elucidated by serially evaluating coagulation and fibrinolytic profiles and complement alterations in patients with HFRS. In the early stage of the disease, platelet counts, platelet survival time, and platelet aggregation in vitro decreased. Prolongation of bleeding time, prothrombin time, and activated partial thromboplastin time was noted, with decreases in coagulation factors II, V, VIII, IX, and X. Levels of fibrinogen were decreased, and those of fibrinogen-fibrin degradation products in serum and urine were increased. Concentrations of plasminogen, alpha 2-plasmin inhibitor, and antithrombin III in plasma were depressed. Procoagulant activity was present in plasma. Circulating immune complexes were found, whereas serum levels of C3 were decreased. In the early stage of HFRS, thrombocytopenia, defects in platelet function, and disseminated intravascular coagulation may play central roles in the pathogenesis of bleeding manifestations. Vasculopathy and immunologic aberrations also may play a role.