The effects of heterotopic bone formation on hip function after arthroplasty was studied in 145 cases of total hip arthroplasty. Hip muscle strength was determined 1.8-2.9 years after the operation, using a Cybex II dynamometer. Heterotopic bone formation was seen after 75% of the operations, and in 21% significant amounts developed (Brooker's classes III and IV). The gain in range of motion after surgery was significantly less in the groups with class III or IV heterotopic bone than in those without heterotopic bone formation. Heterotopic bone did not cause pain or Trendelenburg's limp after surgery; in fact, patients with trochanteric pain had less heterotopic bone than those without this pain. Hip flexion strength was greater in men with heterotopic bone than in those without heterotopic bone. Also, men with heterotopic bone formation had a higher mean maximum strength in extension but lower mean strength in abduction than did men without heterotopic bone (NS). When only patients with unilateral hip disease were considered, the same differences were found around the healthy hip; men with heterotopic bone formation had greater strength in flexion and extension than men without heterotopic bone formation and the same tendencies were seen in women. Accordingly, heterotopic bone had no serious impact on hip muscle strength in this study.