Circulating autoantibodies against the Goodpasture antigen (alpha 3 chain of type IV collagen) in the glomerular basement membrane (anti-GBM) and anti-neutrophil cytoplasm antibodies (ANCA) are each associated clinically with the development of a rapidly progressive glomerulonephritis. Antibodies with both these specificities coexist in a subset of patients, raising the possibility that they might be a result of cross-reactivity. In this study we have shown that 21% of patients with anti-GBM antibodies also had ANCA, and by using cross-inhibition assays, antigen-specific enzyme-linked immunosorbent assays, and Western blot analysis, these were shown to be two distinct populations of autoantibodies. In patients with both specificities, a greater proportion of the ANCA had specificity for myeloperoxidase (73.5%) than in patients with ANCA alone (36.6%). The presence of ANCA should be ascertained in all patients with anti-GBM disease as the prognosis for these double-positive patients may be dependent on both populations of antibodies.