This study prospectively evaluated 928 patients with 1318 primary total hip replacements for heterotopic ossification (HO). The mean clinical and radiological follow-up was 2.5 years (range 1.5-3.6 years). HO was noted in 44.6% of all total hips replaced. It was graded as mild (Brooker 1) in 29.2%, moderate (Brooker 2) in 10.5%, and severe (Brooker 3 and 4) in 4.2%. The following factors showed a significantly increased risk of HO: hypertrophic osteoarthritis, HO after contralateral total hip replacement, trochanteric osteotomy, lateral or anterolateral approach, previous hip surgery, subtrochanteric femoral osteotomy, and male gender (p < 0.05 in chi-square analysis of independence and multivariable analysis). Patients with rheumatoid arthritis showed less HO. A combination of any of these factors resulted in a significant increase in the risk of developing HO.