The effect of indomethacin 25 mg 3 times daily during the first 2 postoperative weeks in preventing heterotopic bone formation after cemented total hip arthroplasty was investigated in a randomized, double-blind and placebo-controlled clinical trial on 57 patients. 16 patients were secondarily excluded, leaving 19 patients in the indomethacin group and 22 patients in the placebo group. Evaluated from the 3-month radiographs, 18/19 indomethacin patients developed either no or only the milder Grade 1 ossification. In contrast, 11/22 placebo patients developed Grade 2 or 3 ossifications. Our observations favor indomethacin prophylaxis for 2 weeks in cemented arthroplasty of the hip.