Anti-neutrophil cytoplasmic autoantibodies (ANCA) were detected in 12 out of 37 (32%) serum samples from patients with anti-glomerular basement membrane (GMB) disease by an indirect immunofluorescence assay. In 11 cases, ANCA were directed against myeloperoxidase, as revealed employing neutrophils devoid of this enzyme as the test substrate. Patients having both ANCA and anti-GBM antibodies (AGBMA) were considerably older (mean age 59 years) than patients with AGBMA alone (mean age 33 years). In addition, patients with both antibodies had some clinical and pathologic data that suggested an associated systemic vasculitis. This was supported by the fact that among these patients, those with highest ANCA titres recovered renal function despite being initially on hemodialysis, as opposed to those with lowest ANCA titres or AGBMA alone. In patients with both antibodies, there was an inverse relationship between AGBMA and ANCA values (p = 0.02). Moreover, the mean AGBMA level tended to be higher for patients with AGBMA alone than for those with both ANCA and AGBMA. These results suggest that, at least in some cases, there may be a contribution of an ANCA-related mechanism in the pathogenesis of anti-GBM disease. Although the exact role of ANCA in this and other diseases remains to be clarified, there is important clinical evidence that in anti-GBM disease ANCA may represent a serologic marker of good prognosis identifying a subset of patients who may recover renal function.