Three hundred and forty-six consecutive patients undergoing total hip or total knee replacement were prospectively studied to evaluate the effectiveness of intermittent pneumatic compression of the legs for the prevention of postoperative deep vein thrombosis. All patients were serially studied using impedance plethysmography and duplex ultrasound with color flow preoperatively and on the fourth and seventh postoperative day. The incidence of postoperative deep vein thrombosis in this series was 4 percent. Each patient with a postoperative duplex ultrasound had positive impedance plethysmography. Of the 14 patients who had postoperative deep vein thrombosis, seven had positive test results on postoperative day four and seven had positive tests on postoperative day seven. No patients were symptomatic. The results suggest that the high incidence of postoperative deep vein thrombosis after extensive orthopedic operation is significantly lowered by the use of intermittent pneumatic compression. Intermittent pneumatic compression, therefore, may be the preferred approach in prophylaxis of postoperative deep vein thrombosis.