A cyclo-oxygenase (COX)-1 and COX-2 inhibitor (indomethacin) and a selective COX-2 inhibitor (celecoxib) were compared in the prevention of heterotopic ossification after total hip arthroplasty. In 250 patients receiving indomethacin and in 150 patients receiving celecoxib for 20 days after surgery, an overall incidence of heterotopic ossification of 17.5% and 14.3% was seen, respectively (difference not statistically significant: P > .05). No grade III or IV ossifications were seen in either group. Twenty-one patients in the indomethacin group (8.4%) and 3 patients in the celecoxib group (2.0%) required treatment discontinuation, because of side effects (P < .05). Celecoxib, a selective COX-2 inhibitor, shows the same efficacy as indomethacin in the prevention of heterotopic ossification after hip prosthesis with significantly fewer side effects.