Sixteen cases of diabetic glomerulopathy are reported. Direct immunofluorescent and ultrastructural studies of renal biopsy tissues demonstrated that two patients had linear deposits of IgM and C'3 in the absence of IgG, four diabetic patients had sclerosis-induced entrapment of immunoglobulins and complement, and one patient had granular immune complexes in the subepithelial and intramembranous portion of the glomerular basement membrane. In one patient, who had nodular glomerular lesions, diffuse fibrillar deposits of electron-dense material were observed in the mesangium. In this mesangial infiltrate, light microscopy revealed the absence of amyloid and direct immunofluorescence revealed the absence of all immunoglobulins, complement components, and fibrinogen. Our study suggests that the morphologic alterations observed in diabetic glomerulopathy might be mediated by either immune mechanism or by abnormal biochemical or functional factors, such as impairment of the mesangial IgA clearance mechanism.