Renal biopsies were obtained from 164 patients with diabetes mellitus. Their histological changes were evaluated together with clinical findings and prognosis. In 36 patients, various types of glomerulonephritis were complicated: mesangial proliferative glomerulonephritis (17 patients), membranous glomerulonephritis (8), endocapillary proliferative glomerulonephritis (5), membranoproliferative glomerulonephritis (4) and minimal change nephrotic syndrome (2). Superimposed glomerulonephritis was suspected in diabetic patients with a short history of less than 5 years, persistent proteinuria, occasional hematuria and no retinopathy. They may, however, produce little effects on the long-term prognosis of diabetic patients except membranoproliferative glomerulonephritis.