The reported incidence of heterotopic ossification (HO) after total hip arthroplasty (THA) ranges up to 50%. HO causes pain and restricted range of motion (ROM) in a significant number of these THA patients. From 1983 to 1988, 177 primary cemented THAs were performed in 131 consecutive patients. Six hundred fifty milligrams of buffered aspirin administered twice daily for two weeks was used as a prophylaxis for thromboembolic disease. There was an unusually low incidence of HO in this group of patients. Aspirin treatment was instituted the night before surgery and continued for two weeks, except in 13 patients (7%) who had to stop treatment because of gastrointestinal symptoms. All patients had at least one year of roentgenographic study postoperatively. According to the Brooker Classification of HO, there were 169 (96%) Grade I and Grade 0 hips, six (3%) Grade II, two (1%) Grade III, and no Grade IV. None of the patients had symptomatic restriction of ROM attributable to heterotopic bone. Aspirin is a safe and inexpensive agent for prevention of HO after THA.