Eleven cases of mesangial proliferative glomerulonephritis whose kidney biopsies were studied with light, immunofluorescent, and electron microscopy are described. Nine patients presented with nephrotic syndrome, one with proteinuria and hematuria, and one with proteinuria alone. Morphologically mesangial proliferative glomerulonephritis was characterized by diffuse mesangial cell proliferation and some increase in mesangial matrix. On immunofluorescence, mesangial IgM deposition was observed in all cases and was considered a distinct feature of mesangial proliferative glomerulonephritis. Electron microscopy showed electron-dense granular deposits within the mesangial matrix in four cases. The clinical course was variable. Of the eight cases with nephrotic syndrome, four treated with steroids alone and four treated with steroids and cytotoxic drugs, one in each group achieved remission while the remaining patients continued to have steroid dependency or resistance. Two of these latter patients manifested steroid responsiveness, steroid resistance, and spontaneous remission at different times in their courses. Renal function remained normal in all. These cases demonstrate that mesangial proliferative glomerulonephritis is an entity characterized by increased mesangial cellularity, deposition of IgM in a mesangial distribution, a relatively benign course, and variable response to treatment.