A prospective, randomized study of the effectiveness of intraoperative and postoperative use of intermittent pneumatic compression, alone or in combination with oral administration of either aspirin or low-dose warfarin, was done of a consecutive series of patients who had a total hip replacement and were more than thirty-nine years old. All patients began walking by the third postoperative day. One hundred and ninety-six patients who had 217 total hip arthroplasties were included. Twenty-eight per cent of the procedures were revisions of a previous total hip replacement or of an endoprosthesis, and the remainder were primary arthroplasties. Patients were randomized as to the type of prophylaxis that they received: intermittent pneumatic compression alone, seventy-six hips; intermittent pneumatic compression and aspirin, seventy-two hips; or intermittent pneumatic compression and low-dose warfarin, sixty-nine hips. Before discharge from the hospital, and at an average of seven days after the operation, all patients were evaluated for the presence of proximal deep-vein thrombosis with either venography on the side of the operation or with bilateral venous ultrasonography. The relative frequency with which thrombosis occurred in a proximal vein was not significantly different in the three groups; the over-all relative frequency was 10 per cent. Intermittent compression during and after the operation effectively reduces the rate of proximal-vein thrombosis after total hip replacement. With the number of patients in our study, the effectiveness of this technique could not be shown to be augmented by oral administration of either aspirin or low-dose warfarin.