Vasculitis comprises a broad group of syndromes characterized by inflammation and necrosis in the walls of blood vessels, resulting in narrowing or occlusion of the lumen. The distribution of blood vessel involvement varies considerably and serves as the basis for one classification of the vasculitic syndromes: large vessels (Takayasu arteritis, giant-cell arteritis); medium and small muscular arteries (polyarteritis nodosa, Churg-Strauss syndrome, Wegener's granulomatosis, vasculitis in rheumatic diseases); and small vessels ("hypersensitivity" vasculitis, Henoch-Schonlein purpura, microscopic polyangiitis, cryoglobulinemia). The pathogenesis of most forms of vasculitis is only beginning to be understood, but is probably varied and complex. Because of the variability of vasculitides, there is no single, uniform method of diagnosis and treatment. However, a detailed history, careful physical examination, and appropriate laboratory tests are needed in all cases to determine the type of onset, course of illness, organ systems affected, and extent of involvement. Biopsy examination of involved tissues is nearly always essential for diagnosis, except in Takayasu arteritis, when aortography is usually indicated. In most cases of vasculitis, corticosteroid therapy is necessary. When the organ involvement is broader and more progressive (e.g., in cases of polyarteritis nodosa or Wegener's granulomatosis), a combination of cytotoxic drug and corticosteroids is often needed.