A quantitative morphological study, at light microscopical level, of hearts from young diabetics and nondiabetics was performed. The groups were comparable with respect to sex-ratio, age, heart weight and blood pressure. Findings in the hearts were as follows: In arterioles in which the luminal diameter ranged between 15-50 mu, a strongly PAS-positive stained vessel wall occurred at a higher frequency among diabetics than among non-diabetics (75 per cent respectively 33 per cent). The PAS-positive structures, however, occupied the same area of the vessel wall in the two groups. In arterioles from diabetics, the number of cells in tunica media was increased as compared with that in non-diabetics (2 p less than 0.01). The amount of perivascular connective tissue was also increased in the diabetics (2 p less than 0.01). There was no indication of an endothelial cell proliferation in the PAS-positive stained vessels from diabetics and non-diabetics. The wall of the capillaries was not thickened and it was not more PAS-positive in the hearts from diabetics than in those from non-diabetics. Moreover, the number of capillaries per square millimeter of heart muscle was the same in the two groups of hearts. In the present light microscopical study, the diabetic micro-angiography of the heart was demonstrable in the arterioles. The capillaropathy known to occur in other organs was not present in the heart muscle from patients with diabetes of long standing.