The endothelium modulates vascular tone by releasing NO, which is a potent vasodilator and inhibitor of platelet aggregation. Thus, the endogenous nitrate has an important protective role in preventing vasospasm and thrombus formation. In addition, the endothelium is a source of contracting factors, such as endothelin-1. Due to its strategical anatomic position, the endothelium is a primary target for injurious stimuli and cardiovascular risk factors. Oxidized LDL reduce the endothelial production of NO and enhance that of endothelin-1. The same pattern of endothelial dysfunction occurs in hypercholesterolemia and in part in atherosclerosis. These alterations of endothelial function may contribute to vasospasm and thrombus formation, which are common events in patients with atherosclerosis.