Anti-neutrophil cytoplasmic autoantibodies (ANCA) react with constituents of neutrophil primary granules and monocyte lysosomes. Indirect immunofluorescence microscopy using alcohol-fixed neutrophils demonstrates two ANCA types: one causing cytoplasmic staining (C-ANCA), and a second causing artifactual perinuclear staining (P-ANCA) that frequently has specificity for myeloperoxidase. Using indirect immunofluorescence microscopy (IIFM) and enzyme immunoassays (EIA), sera from over 300 patients with renal disease, with and without systemic vasculitis, were analyzed. Of 76 patients with pauci-immune glomerulonephritis with crescents or necrosis, 87% had ANCA by IIFM (38% of C-ANCA type, 49% of P-ANCA type), and 78% had ANCA by EIA. Of 55 patients with nonlupus immune complex-mediated glomerulonephritis, only 11% had ANCA by IIFM and 5% had ANCA by EIA. Of 24 patients with anti-GBM antibody-mediated glomerulonephritis, none had ANCA. Renal and extrarenal lesions were studied in 81 patients with ANCA-associated glomerulonephritis. These patients formed a pathologic continuum ranging from renal-limited to widespread systemic vascular injury, including patients with primary crescentic glomerulonephritis, Wegener's granulomatosis, and polyarteritis nodosa. In ANCA-positive patients the frequency of C-ANCA and P-ANCA correlated with disease distribution. P-ANCA was most frequent with renal-limited disease and C-ANCA was most frequent when there was lung and sinus involvement. It is proposed that ANCA are not only useful diagnostic markers, but may also be directly involved in a novel pathogenetic mechanism that is a frequent cause of crescentic glomerulonephritis and systemic necrotizing vasculitis.