Six diabetic patients with either a nephrectomy or a congenitally single kidney, and who later developed diabetic nephropathy, are described. They represent about 3% of our diabetic population with nephropathy, and are the only patients in our clinic population known to have single kidneys. A reduced renal mass may augment the renal glomerular hyperfiltration associated with diabetes mellitus and increase the risk of later developing nephropathy.