A group of 101 diabetic patients containing equal numbers of patients with and without clinical atherosclerosis and 104 control subjects of similar age and sex were studied. The diabetic patients with atherosclerosis were found to have higher triglyceride and cholesterol levels, higher insulin-glucose ratios, and a higher frequency of pre-β band staining on lipoprotein electrophoresis than diabetic patients without atherosclerosis or control subjects. Diabetic patients with atherosclerosis could be discriminated better from those without atherosclerosis by the use of triglyceride levels than by the use of cholesterol levels. Segregated arbitrarily into three categories of body weight, diabetic patients with atherosclerosis had higher triglyceride levels than similarly classified diabetic patients without atherosclerosis. Basal insulin levels, triglyceride levels, and indices of body weight were highly intercorrelated in the diabetic patients with atherosclerosis. However, fasting levels of serum insulin were similar in the two groups of diabetic patients. There was no clear relationship