We prospectively studied the use of either aspirin or intermittent pneumatic compression (IPC) as prophylaxis against thromboembolism after 330 consecutive total hip arthroplasties. Duplex ultrasonography of the veins of both lower extremities and ventilation-perfusion lung scans were done preoperatively and 7 to 14 days postoperatively. Eight patients in the IPC group (5%) had asymptomatic deep vein thrombosis; there were no symptomatic thrombi. In the aspirin group, 10 patients (7%) had asymptomatic deep vein thrombosis and two patients (1%) had symptomatic deep vein thrombosis. This difference was not statistically significant. There were no fatal pulmonary emboli in either group. In the IPC group, only one patient had symptomatic pulmonary embolism and 20 patients (22 hips [12%]) had asymptomatic pulmonary embolism. In the aspirin group, two patients (1%) had symptomatic pulmonary embolism and 26 patients (18%) had asymptomatic pulmonary embolism. This difference in asymptomatic pulmonary embolism between the two groups was statistically significant. Both groups had a low incidence of deep vein thrombosis, as shown by Duplex ultrasonography, but IPC was more effective than aspirin in preventing asymptomatic pulmonary embolism after total hip arthroplasty.