ABSTRACT. Objective. To determine whether "continuous" celecoxib is more efficacious than "intermittent" use in preventing osteoarthritis (OA) flares of the knee and/or hip. Methods. A double-blind, randomized, multicenter international study comparing efficacy and safety of continuous (daily) versus intermittent (as required during predefined OA flare) celecoxib 200 mg/day in 858 subjects, aged 18-80 years. The study consisted of 3 periods: (I) screening/washout visit; (II) open-label run-in with celecoxib; and (III) 22-week blinded treatment. Only subjects whose OA flares resolved in Period 2 (without subsequent flare) were randomized. The primary endpoint, number of flares per time of exposure during Period III (number of flares per month), was compared using analysis of variance with treatment as the independent variable. Acetaminophen was available as rescue medication. Results. Of 875 subjects randomized to treatment, 858 subjects received treatment. At randomization > 70% were female; mean age 58.6 years; mean disease duration 6.5 years; total Western Ontario and McMaster Universities Osteoarthritis Index mean score 25.8; ~45% had hypertension; and ~20% were using aspirin (for cardiovascular prophylaxis). Subjects receiving continuous treatment reported 42% fewer OA flares/month than intermittent users (p < 0.0001) or 2.0 fewer OA flares over 22 weeks. Statistical and clinically meaningful benefits in secondary outcomes were also evident with continuous treatment. There were no differences in adverse events (AE) or newonset/aggravated hypertension. Conclusion. Continuous treatment with celecoxib 200 mg/day was significantly more efficacious than intermittent use in preventing OA flares of the hip and knee, without an increase in overall AE, including gastrointestinal disorders and hypertension, during 22 weeks of treatment