This is a multidisciplinary study of "idiopathic" systemic vasculitis in man with the following research objectives: (1) defining the immunopathogenic mechanisms involved in the initiation and perpetuation of vascular inflammation, (2) identifying specific etiologic factors such as infectious agents or drugs which may trigger the immunopathogenic mechanisms, (3) employing immunohistologic techniques and assays for localizing and quantitating immune complexes which appear to be central in the disease process.