Vascular endothelial dysfunction describes a phenotype prone to atherogenesis and clinical complications of this disease process. Endothelium-dependent vasodilator function, reflecting local bioavailability of nitric oxide, can be measured clinically in the peripheral and coronary circulation and corresponds with other measures of endothelial biology including inflammatory status and thrombotic tendency. Although conventional risk factors are key determinants of endothelial dysfunction, many other factors, including the individual's genetic profile, also appear to exert important positive and negative functional influences. Thus, endothelial vasodilator function can be regarded as an integrated index of all atherogenic and atheroprotective factors acting on the vascular wall, reflecting underlying biology and inherent atherosclerotic risk. The potential clinical utility of endothelial vasomotor testing as a prognostic tool in risk assessment and for the monitoring of therapy requires further validation before recommending its wider routine use.