Whether long-term glycemic control will prevent the chronic vascular complications of diabetes mellitus remains unknown. Microangiopathy and accelerated macroangiopathy are prevalent in both type I, or insulin-dependent diabetes mellitus, and type II, or non-insulin-dependent diabetes mellitus. Microangiopathy is predominantly responsible for the excessive morbidity and mortality in type I diabetic patients, whereas accelerated macroangiopathy directly relates to the excessive morbidity and mortality in type II diabetic patients. Institution of euglycemia for short periods will reverse preclinical, functional, renal, and retinal abnormalities, but will not reverse clinical nephropathy and retinopathy. Intensive insulin therapy, although it increases the risk of hypoglycemic encephalopathy, seems rational for type I diabetic patients without vascular complications who can recognize and respond normally to hypoglycemia. In patients with type II diabetes, sulfonylurea therapy, which is associated with fewer adverse reactions than intensive insulin therapy, may lower the risk of atherosclerosis development by correcting hyperglycemia and associated lipid abnormalities.