Twenty consecutive primary noncemented total hip arthroplasties performed on 12 men with ankylosing spondylitis were available to be studied clinically and radiographically at an average follow-up period of 75 months (range, 27-121 months). The average patient age at the time of surgery was 35 years (range, 23-53 years). Harris hip scores averaged 48.4 before surgery and 89.1 at the most recent follow-up examination. Significant improvements in pain, function, and range of motion were observed following total hip arthroplasty. No hip has required a surgical revision or reoperation. Heterotopic ossification was observed in 6 of 14 hips (43%) in the ankylosing spondylitis patients who had not had any perioperative prophylaxis for heterotopic bone formation. By contrast, 43 of 49 hips (88%) demonstrated heterotopic bone formation in a well-defined control group of 45 men with other diagnoses undergoing the same procedure by the same group of surgeons at the same institution. During the same period using the same implants. A thorough review of the literature and data from the institution does not support the notion that ankylosing spondylitis patients are necessarily predisposed to form heterotopic ossification. The use of routine perioperative prophylaxis for heterotopic ossification may not be warranted in all patients with ankylosing spondylitis undergoing routine primary noncemented total hip arthroplasty.