A study of 95 patients determined the incidence of postoperative venous thrombosis and the effectiveness of simple prophylactic measures. In addition, the accuracy of the radioactive fibrinogen technique for the detection of thrombosis was compared to venography. Deep venous thrombosis was detected in six of 44 (14%) control patients. However, only two of 51 (4%) patients on whom prophylactic measures were applied developed thrombosis. The ages of the patients with thrombosis occurring postoperatively were significantly higher than the ages of the other patients in this study. There was an excellent correlation of the results obtained by the radioactive method with those of venography. Laboratory tests indicated a deficient endogenous fibrinolytic activity in those patients who developed thrombosis.