The discovery of nitric oxide (NO) as a crucial endothe-lium-derived molecule for vascular relaxation and the recognition of the endothelium as more than a passive interface between blood and the vessel wall led to substantial progress in the field of vascular research. 1 Endothelial dysfunction is a pathological condition characterized mainly by an imbalance between substances with vasodilating, antimitogenic, and antithrombogenic properties (endothelium-derived relaxing factors) 2 and substances with vasoconstricting, prothrombotic, and proliferative characteristics (endotheliumderived contracting factors). 3 Among the most important vasodilator molecules, particularly in muscular arteries, is NO, which also inhibits other key events in the development of atherosclerosis such as platelet adhesion and aggregation, leukocyte adhesion and migration, and smooth muscle cell proliferation. Particularly in the microcirculation, prostacyclin and endothelium-derived hyperpolarization factors (an umbrella term for substances and signals hyperpolarizing vascular myocytes by opening voltage channels4) also play an important role.

Generally, loss of NO bioavailability indicates a broadly dysfunctional phenotype across many properties of the endothelium. Thus, the assessment of its vasodilator properties resulting from NO and other molecules may provide information on the integrity and function of the endothelium. Interestingly, most, if not all, cardiovascular risk factors are associated with endothelial dysfunction, 5 and risk factor modification leads to improvement in vascular function. Endothelial dysfunction has been detected in the coronary epicardial and resistance vasculature and in peripheral arteries, so endothelial dysfunction can be regarded as a systemic condition. 6 Importantly, the process of atherosclerosis begins early in life, and endothelial dysfunction contributes to