We studied the effect of indomethacin on the prevention of formation of heterotopic bone after total hip replacement. In a randomized, double-blind clinical trial involving 201 patients, 102 patients received twenty-five milligrams of indomethacin three times daily for the first six postoperative weeks, and the other ninety-nine patients received a placebo. One year after the operation, eighty-nine of those who had received indomethacin had no sign of heterotopic ossification, and the remaining thirteen had a grade-I lesion. In the group that had received a placebo, twenty-seven had no heterotopic ossification; twenty-four, a grade-I lesion; thirty, a grade-II lesion; and eighteen, a grade-III lesion. Significantly fewer patients who had received indomethacin had formation of heterotopic bone compared with those who had been given a placebo (chi-square test, p less than 0.0005). Only patients who had grade-III formation of heterotopic bone had a significant reduction in movement of the hip.