Background: Cellulitis and chronic oedema are common conditions with considerable morbidity. The number of studies designed to assess the epidemiology of cellulitis in chronic oedema is scarce. Objectives: To investigate the prevalence and risk factors of cellulitis in chronic leg oedema, including lymphoedema. Methods: A cross-sectional study included 40 sites in nine countries during 2014–17. Adults with clinically proven unilateral or bilateral chronic oedema (oedema > 3 months) of the lower leg were included. The main outcome measures were frequency and risk factors for cellulitis within the last 12 months. Results: Out of 7477 patients, 15·78% had cellulitis within the last 12 months, with a lifetime prevalence of 37·47%. The following risk factors for cellulitis were identified by multivariable analysis: wounds [odds ratio (OR) 2·37, 95% confidence interval (CI) 2·03–2·78], morbid obesity (OR 1·51, 95% CI 1·27–1·80), obesity (OR 1·21, 95% CI 1·03–1·41), midline swelling (OR 1·32, 95% CI 1·04–1·66), male sex (OR 1·32, 95% CI 1·15–1·52) and diabetes (OR 1·27, 95% CI 1·08–1·49). Controlled swelling was associated with a reduced risk (OR 0·59, 95% CI 0·51–0·67). In a subgroup analysis, the risk increased with the stage of oedema [International Society of Lymphology, stage II OR 2·04 (95% CI 1·23–3·38) and stage III OR 4·88 (95% CI 2·77–8·56)]. Conclusions: Cellulitis in chronic leg oedema is a global problem. Several risk factors for cellulitis were identified, of which some are potentially preventable. Our findings suggest that oedema control is one of these. We also identified that advanced stages of oedema, with hard/fibrotic tissue, might be an important clinical indicator to identify patients at particular risk.