Quantitative electron microscopic studies of muscle capillary basement membrane are reported on 154 control subjects and 151 diabetics. In normal subjects and in diabetics significant thickening of basement membrane is observed as an aging phenomenon in both sexes. Significant age-related sex differences in basement membrane width are evident in normal subjects but not in diabetics. Observations on diabetics suggest that basement membrane width is probably normal in most subjects prior to the onset of carbohydrate intolerance and demonstrate that both the magnitude and incidence of basement membrane thickening increase with longer known duration of carbohydrate intolerance. Basement membrane thickening is demonstrable in a significantly higher percentage of subjects over age fifty (63 per cent) than in subjects under age fifty (25 per cent) who have had the disease for four years' known duration or less. The incidence increases to 90 per cent in subjects with complications of retinopathy and/or nephropathy and to 93 per cent in subjects who have had known disease for twenty years or longer. Both morphologic and statistical evidence indicate that thickening of basement membranes related to aging and to diabetes is essentially segmental in character.