Anti-neutrophil cytoplasmic antibodies (ANCA) constitute a new group of antibodies that are directed against myeloid proteins. Several antigens recognized by ANCA have been described. Antibodies directed to proteinase 3, myeloperoxidase and/or elastase are closely associated with necrotizing systemic vasculitis and/or necrotizing and crescentic glomerulonephritis. Changes in levels of anti-proteinase 3 in generally reflect disease activity in Wegener's granulomatosis. The diagnostic significance of other or still unknown specificities has not yet been definitely established. Recent data from in vitro and in vivo models suggest a role for ANCA in the pathogenesis of systemic vasculitides and/or necrotising and crescentic glomerulonephritis.