In a comparison of 30 patients with Behçet's disease and 60 age and sex matched community controls an increased risk of Behçet's disease was associated with tonsillectomy, a history of cold sores, large sibship size, late birth order, travel to countries with high incidence of the disease, and first sexual intercourse before 16 years of age. These findings are consistent with a triggering of the disease by infection during childhood or adolescence in an immunogenetically predisposed host.