The effect of naproxen on heterotopic ossification after total hip replacement was studied in a randomized, double-blind trial. Twenty-eight patients received 250 mg naproxen thrice daily for 4 weeks postoperatively starting on the morning of the operation while 27 control patients received a placebo. Three months after the operation, 13 patients in the control group had heterotopic ossification compared with 4 patients in the group that received naproxen; and after 1 year, the figures were 15 and 4, respectively (P less than 0.01). Three control patients had severe ossifications. We conclude that naproxen given for 4 weeks is sufficient to decrease the incidence of heterotopic ossification after cemented total hip replacement.