Clinical studies were carried out on 526 out-patients with diabetes mellitus on whom the progress of the disease could be followed up for more than 5 years (average 10.3 years). The results obtained were as follows: (1) Ischemic ECC abnormality was noted at a higher frequency in the obese group than in the non-obese group. Even when comparisons were made of the same condition based on the duration of the disease, age, cholesterol level and blood pressure, a similar tendency was observed. (2) Ischemic ECG abnormality was encountered more frequently in the obese group undergoing treatment with insulin or oral hypoglycemic agents, whereas in the non-obese group no difference existed from those undergoing diet treatment. (3) Ischemic ECG abnormality developed in 40.2% of the oral agents group, 40.0% of the insulin group and 9.1% of the diet group in the obese diabetics, while no difference was observed between therapeutic agents in the non-obese diabetics. It might be concluded that a specific state of being obese, or obesity coupled with a state assisting endogenous and exogenous hyperinsulinemia may contribute to the development of atherosclerosis in diabetics.