Myeloperoxidase anti-neutrophil cytoplasmic antibody (MPO-ANCA)- associated glomerulonephritis (GN) is characterized by pauci-immune necrotizing glomerulonephritis(NGN). Although it has been thought that MPO-ANCA is involved in the pathogenesis of capillary injuries in NGN via activation of neutrophils, recent studies suggest a possible role of other factors such as immunoglobulins precipitated on the glomeruli. Here we performed a pathological study investigating a relationship of deposition of MPO, IgG, complements with regard to MPO-positive cells and glomerular capillaries in human MPO-ANCA-associated GN. Renal specimen including 317 glomeruli obtained from 20 patients with MPOANCA- associated GN were analyzed. All of the specimens showed significant focal segmental deposition of IgG. There was a significant glomerular infiltration of MPO-positive cells along with deposition of extracellular MPO in the active lesions of segmental and global NCG, with CD34 staining being decreased in the adjacent areas. IgG deposits were almost colocalized with C3 and partly with MPO, which are also associated with a decrease in CD34 staining, suggesting that immune complex formation and the resultant capillary injuries. Actually occurred, the colocalization of MPO, IgG and C3 was seen only in the glomerular lesions with low severity and activity. These results suggest that not only MPO itself released from the neutrophils but also immune complexes composed of MPO and anti-MPO antibody may play some pathogenetic roles for the glomerular injuries especially in the early phase of human MPO-ANCA-associated GN.