Preoperative and postoperative determinations of whole blood euglobulin clot lysis time (WBECLT) and fibrinogen level were performed in 121 patients undergoing major surgery. The purpose was to delineate the usual response of the fibrinolytic system to surgery and to determine whether changes could be predictive of postoperative thrombotic complications. The usual response is decreased fibrinolysis (increased WBECLT), maximal on first postoperative day with a concomitant marked increase in fibrinogen level, with values slowly returning to normal. A thrombotic complication occurred in seven patients. Six of seven were associated with abnormal postoperative fibrinolytic study results. A WBECLT over 131 minutes or a fibrinogen level over 400 mg/100 ml is abnormal on postoperative day 5. These tests may help clinicians choose patients for heparin sodium therapy.