Antineutrophil cytoplasmic autoantibodies are a useful diagnostic serologic marker for a variety of well-known vasculitic syndromes, including Wegener's granulomatosis, polyarteritis nodosa (especially microscopic polyarteritis nodosa), Churg-Strauss syndrome, and pulmonary-renal syndrome with alveolar capillaritis. Although most patients with antineutrophil cytoplasmic autoantibody-associated disease have systemic disease, disease limited to one organ does occur,eg, isolated necrotizing glomerulonephritis, isolated respiratory tract disease, or isolated orbital disease. Antineutrophil cytoplasmic autoantibody titers may be useful in modulating treatment regimens. There isin vitroevidence that antineutrophil cytoplasmic autoantibodies are directly involved in the pathogenesis of antineutrophil cytoplasmic autoantibody-associated vasculitides.