Disease of coronary, cerebral, and peripheral arteries is associated with exaggerated insulin responses to oral glucose. In three populations, high fasting or post-glucose insulin levels have a predictive value in the incidence of ischemic heart disease and in cardiac mortality. Diabetics who are obese or who have received treatment with insulin have elevated insulin levels and, as a group, have an increased incidence of cardiovascular disease. Insulin, in small concentrations, has effects on arterial tissue including stimulation of smooth muscle cell proliferation and of glucose incorporation into lipid. It is suggested that insulin has a role in the development of atherosclerosis.