Objective. To demonstrate the efficacy and noninferiority of loading dose of celecoxib 200mg compared with 400mg in pain management after endoscopic nasal surgery. Design. A prospective randomized controlled study. Setting. A university hospital. Patients. Consecutive 120 patients undergoing endoscopic nasal surgery under general anesthesia were randomly assigned to three groups of 40 patients each, celecoxib 400mg, celecoxib 200mg, or a control group with no celecoxib. Interventions. Patients received different doses of celecoxib as described 1 hour before anesthesia. Two celecoxib groups received celecoxib 200mg every 12 hours until the fifth day after surgery. Outcome Measures. Noninferiority could be claimed if the lower limit of the confidence interval (CI) for pain score difference was greater than -0.6. At 0, 6, and 48 hours after surgery, prostaglandin E (PGE) and thromboxane A (TXA) to prostaglandin I (PGI) ratios were measured from incision drainage and in plasma. Results. Estimated difference between two loading doses for pain scores over 5 days was 0.275 (95%CI -0.255 to 0.805, P>0.05), indicating the noninferiority of celecoxib 200-400mg. Local PGE at 6 hours and 48 hours positively correlated with pain scores, with correlation coefficients 0.371 (P=0.005) and 0.288 (P=0.033). Systemic TXA to PGI ratios did not differ among groups. Conclusions. An initial dose of celecoxib 200mg was equivalent to celecoxib 400mg with regard to the margin previously specified at -0.6 in reducing moderate postoperative pain in endoscopic nasal surgery both in analgesic efficacy and anti-inflammatory property. One hundred and twenty patients were included in this prospective randomized controlled study. Patients treated with celecoxib had lower pain scores than controls, pain scores correlating with local PGE level. An initial dose of celecoxib 200mg was equivalent to 400mg in reducing moderate pain after endoscopic nasal surgery. Wiley Periodicals, Inc.