Anti-neutrophil cytoplasmic autoantibodies (ANCA) have specificity for proteins in the cytoplasmic granules of neutrophilic and the lysosomes of monocytes. ANCA occur in a high proportion of patients with Wegener's granulomatosis, microscopic polyangiitis (microscopic polyarteritis), Churg-Strauss syndrome and certain forms of drug-induced vasculitis. ANCA with different specificities from those in patients with vasculitis occur in patients with inflammatory bowel disease and rheumatoid arthritis. ANCA titres correlate to a degree with disease activity and response to treatment. ANCA are a useful diagnostic marker but because of the low prevalence of ANCA-associated diseases, their positive predictive value is good only in patients with signs and symptoms of vasculitis. In vitro data indicate that ANCA can activate cytokine-primed neutrophils and monocytes, causing them to degranulate, release toxic oxygen metabolites, adhere to endothelial cells, and kill endothelial cells. If these events occur in vivo, ANCA may be directly involved in the pathogenesis of vasculitis.