Venous thromboembolism after total hip replacement continues to be a serious problem. We conducted a study to determine whether adjustment of the dose of subcutaneous heparin to yield partial thromboplastin times in the high-normal range results in a greater reduction of postoperative deep-vein thrombosis than fixed doses of heparin. Seventy-nine patients undergoing elective hip arthroplasty were randomly divided into two groups two days before surgery. Group 1 (41 patients) received a fixed dose of 3500 IU of heparin subcutaneously ever eight hours; 16 of the 41 (39 per cent) had deep-vein thrombosis diagnosed by venography. Group 2 (38 patients) was started on the same dose, which was then adjusted to keep the activated partial thromboplastin time between 31.5 and 36 seconds. From the day of operation to the eighth postoperative day these patients needed progressively more heparin to maintain the activated partial thromboplastin time in the prescribed range. Only 5 of the 38 (13 per cent) had deep-vein thrombosis (P less than 0.01), and the number of thrombi in proximal veins was also lower in this group (P = 0.003). The number of units of blood transfused, the frequency of postoperative wound hematomas, and the drop in hemoglobin levels were identical in the two groups. Adjusted low-dose heparin prophylaxis appears to be a safe and efficacious method to reduce the frequency of deep-vein thrombosis in patients undergoing total hip replacement.