The purpose of this study was to investigate the incidence of premature arterial aging in diabetes mellitus and to re-examine the relationship between this type of degenerative vascular change and the other features of the diabetic state. A sensitive method was used to measure changes in the elastic properties of large arteries. Twenty-nine of 50 diabetic patients had premature arterial aging. Sixty-two per cent of the patients, with abnormally high rigidity indices had retinopathy and 21 per cent had Kimmelstiel-Wilson's syndrome. The findings suggest that in diabetic patients premature arteriosclerosis is not necessarily secondary to peripheral occlusive angiopathy. The importance of the duration of diabetes in the development of premature arterial aging was demonstrated. An accelerated arteriosclerotic process may occur with late onset of diabetes. The relation between the metabolic and the vascular defects, was not clear cut. The possibility that the vascular anomalies of the diabetic trait might be inherited without the metabolic counterpart was suggested by the discovery of premature arterial aging in 11 of 50 nondiabetic subjects with a family history of diabetes.