Patients with diabetes mellitus have a greater morbidity and mortality from cardiovascular disease than patients without diabetes. Concomitant hypertension and diabetes are associated with even greater risk of coronary disease, atherosclerotic and peripheral vascular disease, and congestive heart failure. In addition, an independent left ventricular dysfunction (diabetic cardiomyopathy) exists in patients with diabetes that may manifest itself initially as abnormalities in diastolic function but ultimately in systolic function. Firm evidence for this outcome exists experimentally, and reversal of systolic and diastolic abnormalities has been noted experimentally. The Diabetes Control and Complications Trial (DCCT) indicated that intensive glycemic control ameliorates microvascular complication of neuropathy, proteinuria, and retinopathy. Little evidence exists for macrovascular complications or for left ventricular dysfunction. Preliminary results of a canine study of glycemic control and left ventricular function are presented. Clinical correlates of this study and its results are meager. Determination of the role that glycemic control plays with regard to left ventricular systolic function and congestive heart failure awaits carefully controlled and designed clinical trials.