Behcet's syndrome (BS) is a multi-systemic vasculitis of unknown aetiology. More than one mechanism seems to be operative in the pathogenesis of Behcet's syndrome, including genetic and environmental factors, causing different manifestations of the syndrome. There are several clues to the role of environmental factors and especially micro-organisms in the pathogenesis. These include clinical findings such as a decrease in the frequency of a positive pathergy reaction with surgical cleaning of the skin before the procedure, the acne-arthritis association carrying similar features to acne-associated reactive arthritis, a higher rate of tonsillectomy, cold sores, late birth order, higher number of siblings, history of travel to countries with a high incidence of BS and earlier age at first sexual intercourse. Moreover, basic research on both viruses and bacteria suggests that micro-organisms may be playing a role, possibly through heat shock proteins and T-cell hypersensitivity. (C) 2011 Elsevier Ltd. All rights reserved.