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Diabetes mellitus and impaired glucose tolerance are linked to increased cardiovascular morbidity and mortality. Vascular disease is directly associated with plasma glucose levels, and reduction of these levels forestalls to a certain extent the vascular complications of diabetes, such as myocardial infarction, nephropathies, and retinopathies. In addition to hyperglycemia, there are other risk factors that play a prominent role, such as hypertension, hyperlipidemia, and genetic factors. Endothelial dysfunction is one of the major factors in the development of cardiovascular disease. The vascular endothelium regulates the blood flow by tightly controlling the coagulation system, cell-cell interaction, and vascular tone. These functions are disturbed in diabetic patients. In diabetics, endothelin-1 levels are increased, leading to vasoconstriction. Endothelin levels are directly related to plasma glucose levels. In addition, the endothelial cell-NO axis is disturbed. NO release and function are impaired. This seems to be dependent upon hyperglycemia and genetic factors. Impaired NO function also results in vasoconstriction. Furthermore, enhanced vascular permeability is seen in diabetics. This appears to be related to impaired endothelial cell relaxation and reactive oxygen species as well as advanced glycosylated end products (AGEs). The complex changes seen in diabetes and even prediabetes are therefore related to numerous derailments related to endothelial dysfunction, and no single therapeutic approach is likely to solve the problem of vascular complications.