These antibodies are specific for antigens in the cytoplasm of neutrophils. The main antigenic targets are proteinase 3(PR3) and myeloperoxydase (MPO) but other targets have been described without determinant conclusions for clinical practice. Staining patterns can be distinguished by an indirect immunofluorescence test (IFI), in which ethanol fixed neutrophils from healthy donors are incubated with patient's sera. Two patterns are distinguished: cytoplasmic pattern (c-ANCA) and perinuclear pattern (p-ANCA). When ANCA are detected by IFI, from a practical point of view, anti -MPO and anti-PR3 antibodies are tested. ANCA have been strongly associated with a spectrum of necrotizing small vessel vasculitides that includes Wegener's granulomatosis, microscopic polyangiitis, Churg and Strauss syndrome, pauci-immune focal necrotizing and crescentic glomerulonephritis. ANCA are a diagnosic marker and useful for the follow-up of the patients. ANCA can be observed in other pathologies: rhumatismal autoimmune diseases, inflammatory gut diseases, after drugs (hydralazine, minocycline, propylthiouracil), after silical exposition, infections (cystic fibrosis, endocarditis, HIV infection). The specificity is different and rarely anti-MPO. The ANCA role for the development of vasculitis is not completely elucidated. Some arguments are against a primary role of ANCA in the development of vasculitis. Certainly, amplification role for neutrophil activation is demonstrated but the primary event responsible of neutrophil activation is not yet defined.