Conventional histology and immunoperoxidase staining for fibrin, immunoglobulins, and complement components were used to look for evidence of cutaneous vasculitis and immune complex deposition in Sweet's syndrome. These features were not identified in any of the 15 cases studied. The lack of any vasculitis emphasises the distinctive character of Sweet's syndrome when compared with certain spontaneous and experimentally induced inflammatory skin lesions, and may imply a similarly distinctive pathogenesis.