5 sets of criteria for diagnosis of Behçet's disease are in use--a problem which has hindered interpretation of different studies and collaborative research. An international study group, which included at least one proponent of 4 of the sets, was formed to derive new, internationally agreed diagnostic criteria for Behçet's disease. Data on 914 patients with Behçet's disease, from 12 centres in 7 countries, were compared with controls from the same centres. The new set of diagnostic criteria--which requires the presence of oral ulceration plus any two of genital ulceration, typical defined eye lesions, typical defined skin lesions, or a positive pathergy test--was simpler to use and had an improved discriminatory performance than its predecessors.