Antineutrophil cytoplasm autoantibodies are useful diagnostic serologic markers for a variety of well-known primary vasculitic syndromes, including Wegener's granulomatosis, microscopic polyangiitis, and idiopathic necrotizing and crescentic glomerulo-nephritis. More recently antineutrophil cytoplasm autoantibodies have been found in other vasculitic syndromes, such as Churg-Strauss syndrome, Henoch-Schönlein purpura, and some nonvasculitic diseases such as rheumatoid arthritis, inflammatory bowel disease, and autoimmune hepatobiliary diseases. There is now evidence to suggest that infection might be an important etiologic factor in the development of antineutrophil cytoplasm autoantibody-associated vasculitides. This link has been strengthened by in vitro data that suggest that antineutrophil cytoplasm autoantibodies are directly involved in the pathogenesis of antineutrophil cytoplasm autoantibody-associated vasculitides.