The aim of this study was to evaluate the utility of the American Pain Society (APS) questionnaire in the assessment of osteoarthritis (OA) pain and to determine the onset of action of celecoxib in the treatment of acute flare pain in patients with OA of the knee or hip. Pooled data from three pivotal, randomized, double-blind, placebo-controlled, 12-week trials of patients with OA who exhibited a flare of disease activity after withdrawal of nonsteroidal anti-inflammatory drug or analgesic therapy were evaluated. Patients completed the APS Pain Measure Questionnaire, which evaluates pain intensity and quality of life, at baseline and daily for the first 7 days of therapy. In addition, patients underwent a range of standard OA assessments to evaluate the analgesic efficacy of celecoxib up to 12 weeks. Three thousand two hundred fifty-eight patients were enrolled in the three studies, of whom 2041 completed the APS questionnaire (1010 received celecoxib, 513 received naproxen, and 518 received placebo). Within the first 24 hours, celecoxib at a dose of 200 or 400 mg/d significantly reduced the amount of acute pain experienced compared with placebo for four of the five measures, and statistical significance for the remaining parameter, “pain in the last 24 hours,” was achieved on day 2. Celecoxib at a dose of 200 to 400 mg/d provided similar efficacy to naproxen at a dose of 1000 mg/d. The pain relief observed with celecoxib was maintained for the APS evaluation period. Long-term efficacy assessments showed the efficacy of 200 mg/d of celecoxib to be continuous and maintained for at least the 12 weeks of the study and that it was equivalent to 400 mg/d of celecoxib and 1000 mg/d of naproxen. This study demonstrates that the APS questionnaire is a useful measure of pain and therapeutic response in OA. Celecoxib furthermore seems to be an effective acute and chronic analgesic in OA.