Cross-sectional.Short episodes of more intense pain are common in knee and hip osteoarthritis (OA), but little is known about such pain flares in response to specific activities.To determine prevalence, severity and clinical correlates of pain flares in response to a repeated sit to stand activity.The analyses included 11,013 patients with knee OA and 3,889 patients with hip OA who completed a 30s chair stand test before starting the Good Life with osteoArthritis in Denmark (GLA:D®) treatment program. Prevalence and severity of pain flares were evaluated by change in self-reported joint pain intensity on a 11-point numeric rating scale (NRS) after the test. Correlates with pain flares (increase in NRS ≥2) were assessed using regression analyses.One in every three patients with knee OA, and one in every five patients with hip OA experienced pain flares (NRS ≥2). Low knee/hip confidence, three or more painful body sites, fewer than 12 chair stands in 30s, and BMI ≥30 were associated with pain flares from the 30s chair stand test in patients with knee and hip OA. Low self-efficacy and joint stiffness was associated with pain flares in patients with knee OA. Using pain medication was associated with pain flares in patients with hip OA.Pain flares in response to a repeated sit to stand activity were common in patients with knee and hip OA. Clinical correlates, including joint confidence, functional performance and BMI, potentially modifiable with patient education, exercise therapy and weight loss, were associated with pain flares. .