A 38-year-old man was presented with nephrotic syndrome associated with hematuria and mild hypocomplementemia. Renal biopsy revealed lobular mesangial proliferation, thickened capillary walls, and intraluminal protein thrombi. Immunofluorescence showed marked and mild depositions of immunoglobulin (Ig) M heavy chains and complement C3, respectively, in a peripheral lobular pattern. On light chain staining, only kappa (κ) light and IgM heavy chains showed a similar pattern. Electron microscopy showed fine granular electrondense deposits in subendothelial areas and numerous globular deposits (varying size and density) in glomerular capillary lumens. Serum levels of Ig κ, but not of free κ, light chains were significantly increased. Bone marrow aspiration revealed a normocellular marrow. Waldenström's macroglobulinemia and cryoglobulinemia were ruled out. Clinically, steroid therapy was ineffective and proteinuria persisted. This report demonstrates that glomerular deposition of monoclonal IgM-κ can produce membranoproliferative- like changes in the glomeruli. This condition may be recognized as proliferative glomerulonephritis with monoclonal IgM deposits similar to the recently recognized proliferative glomerulonephritis with monoclonal IgG deposits.