Antineutrophil cytoplasmic autoantibodies (ANCA) are a useful diagnostic serologic marker for the most common forms of necrotizing vasculitis, provide a means of categorizing vasculitides so that diagnostically useful shared pathologic and clinical characteristics can be recognized, and offer insight into the pathogenesis of previously idiopathic diseases. ANCA-associated vasculitides can be categorized into a number of distinctive clinicopathologic categories, eg, Wegener's granulomatosis, Churg-Strauss syndrome, pulmonary renal syndrome, microscopic polyarteritis nodosa, leukocytoclastic angiitis, and necrotizing and crescentic glomerulonephritis. At least the latter four syndromes can also be caused by other ANCA-negative immunopathogenic mechanisms, eg, immune complex deposition. Therefore, thorough diagnostic classification requires both an assessment of clinicopathologic category, as well as an assessment of immunopathologic category. Although different ANCA-associated vasculitic syndromes have distinctive clinical and pathologic features, all ANCA-associated vasculitides share a number of common pathologic features, ie, focal distribution, necrosis, and neutrophil infiltration. ANCA assays have very good sensitivity and specificity for ANCA-associated diseases, but the prevalence of these diseases in the patient population being analyzed must be taken into consideration when determining the predictive value of a test result. As with all serologic tests, ANCA results must be integrated with other clinical and pathologic data in order to reach the most accurate diagnostic conclusion.