Until recently, the endothelium was regarded as a relatively inert cell layer. However, over the past 20 years, research has revealed an extraordinary array of endothelial functions, including control over coagulation, fibrinolysis, arterial tone and vascular growth. Importantly, endothelial dysfunction has been implicated as a key event in the pathogenesis of atherosclerosis, coronary vasoconstriction and, probably, myocardial ischemia. The recent demonstration that endothelial dysfunction may be reversible raises the possibility of slowing the progression of atherosclerosis or modifying arterial function, or both, to decrease the risk of acute cardiovascular events.