In 79 patients undergoing hip or knee replacement surgery the preoperative condition of the popliteal vein valves was assessed by Doppler ultrasonography. Presence of postoperative deep vein thrombosis was determined by bilateral ascending phlebography seven to ten days after surgery. Antithrombotic prophylaxis consisted solely of TED stockings and early mobilization. The overall incidence of deep vein thrombosis was 20%. All thrombi were calf vein thrombi. One of these thrombi extended into the popliteal region and two into the femoral region. The incidence of deep vein thrombosis in patients with preoperative popliteal vein reflux when compared with patients with a normal popliteal valvular function was 55-15% (p less than 0.01). The incidence of deep vein thrombosis was also significantly lower in patients operated under epidural anaesthesia compared with patients under general anaesthesia. It is concluded that valvular incompetence of the popliteal vein predisposes to postoperative deep vein thrombosis.