BACKGROUND: We aimed to describe the main features of Behçet's disease (BD) in children in the largest prospective cohort to date and to propose a classification. METHODS: An international expert consensus group was formed to define a data set of minimal symptoms for the inclusion of patients. Patients were entered prospectively during 66 months. Experts classified patients on a consensus basis. The concordance of two international classifications was analysed in confirmed patients with BD. Comparisons of subgroups of patients helped define consensus criteria. BD-associated clinical manifestations were also investigated in three control diseases extracted from an independent data set (Eurofever). FINDINGS: In total, 42 centres from 12 countries included 230 patients; data for 219 (M/F ratio=1) could be analysed. The experts classified 156 patients (71.2%) as having confirmed BD. Males more often than females showed cutaneous, ocular and vascular symptoms and females more often genital aphthosis. Age at disease onset and skin and vascular involvement were lower for European than non-European children. Oral aphthosis was the presenting sign for 81% (179/219) of patients. The mean delay to the second symptom was 2.9±2.2 years. International classifications were not concordant with the expert classification. Our paediatric classification contains six categories, a minimum of three signs (each in a distinct category) defining paediatric BD. Three clinical signs discriminated our cohort from the Eurofever cohorts. INTERPRETATION: We present a comprehensive description of a large cohort of patients from both European and non-European countries and propose the first classification of paediatric BD for future therapeutic trials.