We report immunopathologic findings observed in renal biopsies of 18 patients with diabetic glomerulosclerosis and associated glomerulonephritis. 10 patients had nodular and 8 patients diffuse diabetic glomerulosclerosis. Focal proliferative glomerulonephritis (4 patients), membranous glomerulonephritis (3 patients), postinfectious glomerulonephritis, Henoch-Schönlein nephritis, IgA nephropathy of Berger (1 patient each), and mesangiopathic glomerulonephritis (8 patients) were the superimposed conditions. In patients with diabetic renal disease, a rapid loss of renal function or an atypical urinalysis could be the expression of a superimposed immune complex disease. Because of functional and biochemical alterations, the glomeruli of diabetic patients may be more susceptible to immune-mediated injury.