Immunoelectron microscopy was used in this study of IgA nephropathy to examine the relationship between immune deposits and electron-dense deposits seen by electron microscopy, and these findings were correlated with the severity of mesangial proliferation. Immunoelectron microscopic studies for antihuman γ-chain, α-chain, µ-chain, C3, and C3d were performed by the method of Nakane in 16 patients with IgA nephropathy. The patients with minimal glomerular involvement and focal prohferative glomerulonephritis showed electron-dense reaction products of IgA in the paramesangial area, while the patients with diffuse prohferative glomerulonephritis showed electron-dense reaction products of IgA throughout the enlarged mesangial matrix. Immunoelectron microscopy showed electron-dense reaction products of IgA at the same locations as the electron-dense deposits seen on electron microscopy. C3 deposits were identified in 15 out of the 16 patients, but were less dense than IgA deposits. Electron-dense reaction products of C3 contained both positive and negative sites in 7 patients. Extensive C3d deposits were found in all cases in association with IgA deposits. The locations of these deposits were the same as those of the IgA deposits. These findings suggest that C3 deposits dissociate from the immune deposits and that the locations of the immune deposits correlate well with the severity of mesangial proliferati