nti-neutrophil cytoplasmic antibodies, as detected by indirect immunofluorescence, have limited diagnostic significance as they occur in a variety of inflammatory disorders. The presence of antibodies to defined target antigens of anti-neutrophil cytoplasmic antibodies, that is proteinase 3 and myeloperoxidase, is, however, highly specific for one of the systemic vasculitides, in particular Wegener's granulomatosis, microscopic polyangiitis, Churg-Strauss syndrome, and idiopathic pauci-immune necrotizing crescentic glomerulonephritis. In general, anti-proteinase-3-positive patients show more widespread organ involvement, more granuloma formation, and a more severe initial course of their renal lesions than anti-myeloperoxidase-positive patients; however, there is considerable overlap, and either antibody specificity may be found in the different clinical syndromes. In vitro, anti-neutrophil cytoplasmic antibodies are able further to activate pre-activated neutrophils and monocytes, which can result in endothelial damage. A direct activating effect of anti-neutrophil cytoplasmic antibodies on endothelial cells has been suggested, but those studies should be confirmed. In vivo, experimental data support a pathogenetic role for anti-neutrophil cytoplasmic antibodies, particularly anti-myeloperoxidase, but besides anti-neutrophil cytoplasmic antibodies a second pro-inflammatory stimulus seems to be required to induce lesions. Whether anti-neutrophil cytoplasmic antibodies can be a direct target for treatment has still to be proved. Current immunosuppressive treatment regimens for the anti-neutrophil cytoplasmic antibody-associated vasculitides are, however, unsatisfactory because of side-effects, that is opportunistic infections and malignancies. New treatment regimens, based on new pathogenetic concepts, are currently being tested.