In order to delineate groups of patients suitable for treatment to prevent heterotopic bone formation (HBF) following total hip replacement, 99 patients were examined to evaluate predisposing factors. One year after surgery, HBF was found in 73% of the patients. A significantly increased frequency of HBF was found among men. There was no correlation between age, severity of osteoarthritis, size of osteophytes, or preoperative hip movement and HBF. Previous ipsilateral hip surgery did not increase the risk of HBF. Although not significant, all patients who developed heterotopic bone after previous ipsi- or contralateral hip surgery showed HBF of the same or even a higher grade after the present replacement.