Arterial hypertension is a very common disease and an important risk factor for cardiovascular disease. Patients with arterial hypertension are characterized by functional and structural vascular abnormalities. Vascular endothelium plays a fundamental role in modulating vascular tone and structure. The physiological production of the relaxing factors including nitric oxide, prostacyclin and hyperpolarizing relaxing factors protects the vessel wall by antagonizing the first pathogenetic steps of atherosclerosis and thrombosis. Endothelial cells may also produce endothelium-derived contracting factors. The principal component of these contracting factors is endothelin-1, which promotes the growth of the smooth muscle cells and has a vasoconstrictive and blood pressure raising effect. Defective nitric oxide production is already detectable in normotensive offspring of hypertensive patients and young essential hypertensives. A dysfunctional endothelium due to reduced nitric oxide availability associated with an increased production of oxidative stress and vasoconstricting factors is considered as an early indicator of atherothrombotic damage and of cardiovascular events also in patients with arterial hypertension. Moreover, patients with arterial hypertension are also characterized by increased arterial stiffness. This parameter, known as a sign of cardiovascular risk since the 19th century, has been shown to be a predictor of adverse cardiovascular outcome and its measurement in hypertensive patients is suggested by the European guidelines for the diagnosis and treatment of hypertension.