Two patients with long-standing diabetes mellitus and diabetic retinopathy were evaluated for declining renal function and heavy albuminuria. Initially, diabetic glomerulosclerosis was suspected as the cause of progressive glomerulopathy. However, in both patients the rate of loss of glomerular filtration rate was greater than that usually seen in diabetic glomerulosclerosis, and the urine sediment contained many RBC casts. These findings led to renal biopsy, which demonstrated crescentic glomerulonephritis superimposed on diabetic glomerulopathy. Both patients were treated with prednisone and cyclophosphamide and both experienced substantial improvement in renal function. These experiences demonstrate the importance of searching for evidence of a superimposed treatable glomerulopathy in the diabetic patient with glomerulopathy and advancing renal insufficiency.