WE PERFORMED HEMOSTATIC studies on eight patients undergoing clipping of unruptured cerebral aneurysms to assess the influence of intracranial surgery itself on hemostasis. Blood samples were collected from each patient 10 times: before and after the induction of anesthesia and 6, 12, and 24 hours and 2, 3, 5, and 7 days immediately after surgery. The changes and our interpretation of them include the following: 1) the elevation of thrombin antithrombin III complex levels (activation of blood coagulation) was transient and monophasic; 2) the elevation of plasmin α2-antiplasmin complex and D-dimer levels (activation of fibrinolysis) was biphasic, despite the monophasic elevation of tissue plasminogen activator or plasminogen activator inhibitor-1 levels; 3) the elevation of β-thromboglobulin and platelet-factor-4 levels (activation of platelet) was also biphasic; 4) fibrinogen level and α2-antiplasmin activity increased in the acute phase of the postoperative course (acute phase reaction); 5) the changes in hematocrit appeared to parallel those in various other parameters, especially platelet count, antithrombin III, and plasminogen levels for 1 or 2 days after surgery; 6) fibronectin appeared to be consumed in the acute phase of postoperative course; and 7) general anesthesia did not significantly affect hemostasis. These serial changes seem to be related to the activation of hemostatic systems after intracranial surgery and the subsequent acute phase reaction.