&NA;In 19 patients with chronic subdural hematoma, coagulation and fibrinolysis in venous blood taken at the time of surgery and in the hematoma contents aspirated from chronic subdural hematoma were studied. Compared with coagulation results for venous blood, the hematoma contents demonstrated marked prolongation of the recalcification time, prothrombin time, and activated partial thromboplastin time, and marked reduction of clotting factor V, the hepaplastin test, prothrombin, and fibrinogen. Antithrombin III was also decreased, and fibrinopeptide A was increased in the hematomas. Fibrinolytic results demonstrated that both plasminogen and &agr;2‐plasmin inhibitor were decreased, and both fibrinopeptide B&bgr; 15‐42 and fibrin and fibrinogen degradation products were increased in the hematomas. These findings indicate excessive activation of the clotting system, thrombin generation, and increased fibrinolytic activity occurring in the hematomas. From these results, excessive activation of both the clotting and fibrinolytic systems is emphasized to be the possible etiological factor for the origin and development of chronic subdural hematoma. (Neurosurgery25:25‐29, 1989)