Renal diseases other than diabetic nephropathy were found in 10 of 122 diabetic patients who underwent renal biopsy between 1960 and 1982. These diseases included lupus glomerulonephritis, acute post-streptococcal glomerulonephritis, membranoproliferative glomerulonephritis (type I), focal glomerulosclerosis, idiopathic membranous nephropathy, and nonspecific immune complex glomerulonephritides. Because some of these disorders can alter the management and prognosis of renal disease in diabetic patients, the appearance of urinary abnormalities or deterioration in renal function inconsistent with the natural history of diabetic nephropathy raises the possibility of a nondiabetic renal disease and should lead to a more detailed evaluation.