The diagnosis and classification of vasculitis has been revolutionized by the discovery and characterization of serum antineutrophil cytoplasmic autoantibodies. These autoantibodies are highly specific, objective markers for the major subset of vasculitis that includes Wegener's granulomatosis, polyarteritis nodosa (microscopic polyangiitis), Churg-Strauss syndrome, primary or idiopathic pauciimmune necrotizing and crescentic glomerulonephritis with or without pulmonary hemorrhage, as well as some poorly characterized and overlapping forms of these vasculitides. The finding of antineutrophil cytoplasmic antibodies throughout this group identifies these syndromes as a single category or spectrum of disease. The sensitivity of antineutrophil cytoplasmic antibodies for this group of conditions is high when there is systemic involvement, as defined by the presence of renal involvement. However, the antibodies are only moderately sensitive markers in limited or localized cases of these vasculitides (without renal involvement), and hence diagnosis of these conditions based on histologic and clinical criteria remains important. Two significant types of antineutrophil cytoplasmic antibodies have been identified: anti-proteinase 3 and antimyeloperoxidase antibodies. Both have proven to be of diagnostic value for the spectrum of vasculitis listed above. Remarkably, patients with this spectrum of vasculitis have only one or the other of these two types of antibodies.