We have conducted a prospective study of 372 patients with well-defined forms of systemic vasculitis and connective tissue diseases to determine the prevalence, the antigenic specificities and the clinical associations of ANCA in such cases. These antibodies were detected by indirect immunofluorescence on ethanol-fixed neutrophils and also by enzyme-linked immunosorbent assay using myeloperoxidase (MPO) as a substrate. In our study, ANCA with a cytoplasmic immunostaining pattern were mainly found in patients with biopsy-proven Wegener's granulomatosis with or without renal involvement and pulmonary hemorrhage. Furthermore, MPO-ANCA strongly correlated with necrotizing glomerular and alveolar capillaritis, mostly in patients having a well-established diagnosis of polyarteritis nodosa.