Heterotopic ossification is a well-recognized complication of total hip arthroplasty. In a retrospective survey of 98 total hip arthroplasties in ankylosing spondylitis performed between 1970 and 1977, 39.8% showed some evidences of heterotopic ossification. Restriction of hip movements was noted in only two. Heterotopic ossification occurred within one year and was higher (64.6%) in patients with bilateral hip replacements if there was evidence of heterotopic bone formation in one hip. The complications were relatively high in hips with previous operations (55%); trochanteric bursitis (50%); and trochanteric detachment (63.3%). No correlation was found between high erythrocyte sedimentation rate and the incidence or extent of heterotopic bone formation.