A retrospective study was done on 109 diabetic patients who had renal biopsies during 1974-1984 to determine factors identifying nondiabetic renal disease in patients with diabetes mellitus presenting with renal dysfunction. Six of 49 (12%) patients with type I and 17 of 60 (28%) with type II diabetes mellitus had other renal diseases, with or without diabetic glomerulosclerosis. Multivariate predictors of other renal disease in type I diabetes mellitus were duration less than 5 years (p less than 0.001), absence of proteinuria (p less than 0.001), and absence of neuropathy (p less than 0.05). In type II diabetes mellitus these were late age of onset (p less than 0.001), absence of neuropathy (p less than 0.05), and Caucasian race (p less than 0.005). Some patients with other diseases appeared to respond to therapy directed at their nondiabetic glomerulosclerosis disease. We emphasize the need to distinguish between the subgroup of diabetic patients with nondiabetic renal disease from the majority who have diabetic glomerulosclerosis alone. The latter group should be spared the discomforts, risks, and costs of a renal biopsy.