Two adult patients with mesangial proliferative glomerulonephritis with diffuse IgM deposition in the glomeruli are reported. Case 1 was a 25-year-old female with nephrotic syndrome who showed complete remission after treatment with prednisolone (PSL). Case 2 was a 46-year-old male with asymptomatic proteinuria who showed incomplete remission (0.5-1.0 g/24 hr) of urinary protein without any medication. In light microscopy, these patients revealed minimal or slight proliferation of glomerular mesangial cells without glomerular sclerosis and crescent formation. Deposition of IgM and C3 was observed in the glomerular mesangial areas and capillary walls by immunofluorescence. Electron-dense deposits were observed in the glomerular mesangial areas in these patients. Mesangial proliferative glomerulonephritis associated with diffuse IgM deposition in the glomeruli appears to have a benign clinical course. It has also been suggested that this disease has variant clinical courses since we recently experienced two other patients with mesangial proliferative glomerulonephritis with focal IgM deposits who showed renal tubular dysfunction or chronic renal failure.