The relation between the severity and duration of diabetes mellitus and the severity of ischemic heart disease is uncertain. The clinical findings and the findings at autopsy were studied in 185 patients with diabetes mellitus of adult onset who ranged in age from 37 to 91 years and had a clinical diagnosis of diabetes established for a few days to 50 years before death. No statistically significant association was demonstrated either by simple correlation or by multivariate regression analysis between the clinically diagnosed severity or duration of diabetes and either the overall coronary disease, the number of diseased vessels or the number of myocardial infarctions. The presence of other expected correlations in the multivariate analysis suggested that the results of this study were not spurious. However, comparison with 185 age- and sex-matched control patients revealed that on the average, diabetic patients have more overall coronary disease (p < 0.002), more diffuseness of coronary disease (p < 0.005), more coronary collateralization (p < 0.001), more vessels involved by atherosclerosis (p < 0.001) and more myocardial infarcts (p < 0.001). The results suggest that although diabetes mellitus of adult onset is a condition in which the larger coronary arteries are subject to more atherosclerosis than are those in nondiabetic subjects, the progression of the atherosclerotic disease is unrelated to the duration or severity of the diabetes mellitus.