Antineutrophil cytoplasmic autoantibodies (ANCA) are a serological marker associated with vasculitis and glomerulonephritis. Specifically, ANCA have been found (at frequencies ranging from 70% to 90%) in patients with active Wegener granulomatosis, microscopic polyarteritis nodosa, and idiopathic crescentic glomerulonephritis (with manifestations ranging from kidney-limited disease to extrarenal systemic disease, including pulmonary-renal syndromes). Glomerular lesions in patients with ANCA-associated systemic vasculitis or renal-limited disease are virtually identical. Normal controls are negative for ANCA and <10% of patients with other renal diseases are positive. ANCA may be directly involved in the pathogenesis of the vascular injury that causes the clinical manifestations in ANCA-associated disease.