OBJECTIVE: To determine prevalence, severity, and clinical correlates of pain flares in response to a repeated sit-to-stand activity. DESIGN: Cross-sectional. METHODS: The analyses included 11 013 patients with knee osteoarthritis (OA) and 3889 patients with hip OA who completed a 30-second chair-stand test before starting the Good Life with osteoArthritis in Denmark treatment program. Prevalence and severity of pain flares were evaluated by change in self-reported joint pain intensity on an 11-point numeric rating scale after the test. Correlates with pain flares (an increase on the numeric rating scale of 2 points or greater) were assessed using regression analyses. RESULTS: One out of 3 patients with knee OA and 1 out of 5 patients with hip OA experienced pain flares (numeric rating scale of 2 or greater). Low knee/hip confidence, 3 or more painful body sites, fewer than 12 chair stands in 30 seconds, and body mass index of 30 kg/m or greater were associated with pain flares in response to the 30-second chair-stand test in patients with knee and hip OA. Low self-efficacy and joint stiffness were associated with pain flares in patients with knee OA. Using pain medication was associated with pain flares in patients with hip OA. CONCLUSION: Pain flares in response to a repeated sit-to-stand activity were common in patients with knee and hip OA. The clinical correlates associated with pain flares included joint confidence, functional performance, and body mass index, and are potentially modifiable with patient education, exercise therapy, and weight loss, respectively. .