Necrotizing vasculitis is a term used to describe vessel wall necrosis due to neutrophil infiltration. Current evidence strongly suggests that these cells are responding to elaboration of chemotactic factors of the complement cascade released at the site of deposition of immune complexes in the vessel wall. The antibody is usually IgG or IgM (and rarely IgA), but the only antigens identified with even a minimum certainty are the streptococcal M protein, the hepatitis B surface antigen, and Mycobacterium tuberculosis. Vessels may be involved, leading to specific signs or symptoms, in a wide range of organs, but with those of the skin, kidney, joints, and gastrointestinal tract leading the list. Why vessels of different sizes or location become involved in individual patients is unknown. Therapy with nontoxic drugs, such as antihistamines or salicylates, is indicated when the disease is mild, but vital organ involvement may necessitate therapy with systemic corticosteroids and/or cyclophosphamide.