Background: Nonsteroidal anti-inflammatory drugs(NSAIDs) inhibit both cyclooxygenase-1(COX1) and cyclooxygenase-2(COX-2). There have been several trials showing that a specific inhibitor of COX-2 provide efficacy in surgical pain comparable with NSAIDs. In this study, the efficacy and safety of the rofecoxib which specifically inhibits COX-2 were compared with aceclofenac, an NSAID, in patients with pain after orthopedic surgery. Method : A randomized, open, active comparator-controlled trial of 229 adults with orthopedic surgical pain was conducted. Patients were randomized to one of two treatment groups ; 50mg of rofecoxib once daily or 100mg of aceclofenac twice daily. Clinical efficacy and safety were evaluated over 5-day treatment period. Result : Rofecoxib at dosage of 50mg demonstrated efficacy that was clinically comparable to that of aceclofenac as assessed by primary endpoint(TOPAR8) according to predefined comparability criteria. Results from secondary endpoints(SPID8, patient global assessment of response to therapy) were consistent with those of the primary endpoint. All treatments were generally well tolerated. Conclusion : Rofecoxib was well tolerated and provided efficacy that was clinically comparable, according to predefined equivalence margin, to that of 100mg of aceclofenac twice daily in this 5-day study.