Changes in hemostatic parameters following open-heart surgery with cardiopulmonary bypass were studied in twelve patients for two postoperative weeks. No clinical manifestations of hemostatic abnormalities were found in these patients throughout the study period. The prothrombin time and activated partial thromboplastin time were within normal limits. The platelet counts and fibrinogen levels were depleted on the first postoperative day, followed by a rapid return to above normal levels within 7 days. The fibrinogen/fibrin degradation products (FDP) showed a peak elevation on the first day, followed by a gradual decline. In 92% of the patients, the FDP levels became elevated to more than 10 micrograms/ml. Rate of the increase in FDP level was closely reflected by the amount of postoperative blood loss when the loss exceeded 10 ml/kg of body weight during 48 hours postoperatively. The levels of plasmin inhibitors (alpha 1-antitrypsin and alpha 2-macroglobulin) were also measured. the alpha 1 antitrypsin was elevated during the postoperative period. The alpha 2-macroglobulin showed no significant changes. In summary, the hemostatic changes following open-heart surgery included activation of the fibrinolytic system and depletion of coagulation factors immediately after surgery, followed by hypercoagulability due to a rapid return to above normal levels of coagulation factors and plasmin inhibitors. We consider, however, these changes to be part of the homeostatic response following open-heart surgery.