Coagulation factors were studied in 30 fluids aspirated from 25 patients with chronic subdural hematoma. Compared with the normal range for plasma, the hematoma fluids demonstrated a marked reduction in factors II, V, VII, VIII, and X, moderate reduction of factors IX and XI, and slight reduction of XII. Factor VII and IX inhibitors were not present or negligible. Activated protein C and antithrombin III were decreased and fibrinopeptide A was markedly increased. No case had a basic disorder causing these abnormal data spontaneously. The decrease in activated protein C possibly caused the marked reduction of factor VIII, therefore the intrinsic and extrinsic clotting pathways were affected differently. The results show excessive activation of coagulation, predominantly via the extrinsic clotting pathway in hematoma, suggesting its importance in the growth of chronic subdural hematoma.