Antibodies directed against neutrophils (C-ANCA and P-ANCA) are of distinct diagnostic value in systemic vasculitis. In a prospective study, sera from over 700 patients with suspected vasculitis, including over 200 patients undergoing renal biopsy, were examined for antibodies to neutrophil cytoplasmic antigen (ANCA). An indirect immunofluorescence assay on ethanol fixed human neutrophils identified two types of autoantibody: C-ANCA, which produces diffuse cytoplasmic staining and P-ANCA, which produces an artefactual nuclear/perinuclear staining pattern. The diagnosis of patients in whom ANCA of either form was found was established following case note review according to defined diagnostic criteria. Forty of 45 patients whose sera contained C-ANCA at a titre of greater than or equal to 1/40 satisfied the diagnostic criteria for Wegener's granulomatosis or microscopic polyarteritis. Twelve of 30 patients with P-ANCA at a titre of greater than or equal to 1/40 were also classified as having one of these disorders. Seventeen of the remaining 18 patients had immune-mediated disorders with deep organ damage. Of 47 ANCA-positive patients who underwent renal biopsy, 21 had glomerulonephritis associated with Wegener's glomerulonephritis and a further 22 had a necrotizing or crescentic glomerulonephritis. Ethanol fixation is important for discrimination of C-ANCA and P-ANCA. C-ANCA are highly specific for Wegener's granulomatosis and microscopic polyarteritis. In patients undergoing renal biopsy, the presence of these antibodies is highly specific for a necrotizing or crescentic glomerulonephritis. 