The presence of a true vasculitis is difficult to discern in some of the more common manifestations of Behcet's syndrome, like the papulopustular lesions of the skin. On the other hand, a true vasculitis is seen in all vessel sizes in the majority of the patients. The pathogenesis is not yet known. A Th1-type inflammatory reaction is seen like in some other primary vasculitides. However, in contrast to other vasculitides, granuloma formation is absent. Behcet's syndrome is unique among the vasculitides with its differing geographic distribution in disease expression, the distinctly more severe disease among the male, the predominance of venous disease, and the generally abating disease course with lack of associated increased atherosclerosis.