In order to clarify the pathogenesis of anti-myeloperoxidase antibodies (anti-MPO ab) -associated crescentic glomerulonephritis (CRGN), localization of MPO in renal glomeruli was examined. The three patients with CRGN and circulating anti-MPO ab studied included two with pauci-immune CRGN and one with Goodpasture's syndrome with anti-glomerular basement membrane (GBM) ab as well as anti-MPO ab. Three patients with CRGN without circulating anti-MPO ab were also studied. MPO was detected along the glomerular capillary wall and in the cytoplasm of recruited neutrophils and mononuclear cells in the three patients with anti-MPO ab -associated CRGN. MPO was not detected along the glomerular capillary wall in the three patients with CRGN without anti-MPO ab. It was suggested that MPO, which is induced by neutrophil activation by anti-MPO ab, might have an important role in the pathogenesis of anti-MPO ab-associated CRGN.