108 patients scheduled for hip arthroscopy due to femoroacetabular impingement were randomized to receive either naproxen 500mg or placebo twice daily for the first 3 weeks postoperatively. The purpose of this study was to evaluate if non-steroidal anti-inflammatory drug (NSAID) prophylaxis with naproxen significantly reduced the incidence of heterotopic ossification over 1-year follow-up. Planned interim analysis of 3- and 6-month data of the first 66 enrolled patients demonstrated a significantly lower incidence of heterotopic ossification in the NSAID group (2/32) compared to the placebo group (12/34), and enrollment was stopped due to ethical reasons. Comparison of data at final follow-up among all included patients also demonstrated a significantly lower incidence of heterotopic ossification in the NSAID group versus placebo group.