After the discovery by Furchgott and colleagues in 1980 that the endothelium plays an obligatory role in acetylcholine-induced vasodilation many investigators have elucidated the role of the endothelium in the regulation of vascular tone. While the sympathetic nervous system serves the organism as a whole, the endothelium appears to act as a local regulator adapting blood flow to local metabolic needs. A variety of endothelium-derived relaxing and contracting factors such as nitric oxide, prostacyclin, endothelium-derived hyperpolarizing factor, endothelin and thromboxane A2 play a role in the endothelium-dependent control of vascular tone. Furthermore, nitric oxide inhibits thrombocyte aggregation and adhesion. Many diseases have been reported to be associated with an impaired endothelium-dependent vasodilation which may contribute to an increased susceptibility to vasospasm, decreased inhibition of thrombus formation and an impaired ability to reduce vascular resistance in ischaemic conditions. In hypertension, hypercholesterolaemia and diabetes mellitus this impairment may be interpreted as an early marker of a process that ultimately will lead to atherosclerosis. The impaired endothelium-dependent vasodilation probably contributes to the increased peripheral vascular resistance in hypertension and heart failure. The role of the endothelium does not seem to be restricted to cardiovascular diseases. Several mediators of inflammation stimulate the endothelium to release nitric oxide, suggesting an important role of the endothelium in the haemodynamic sequelae of sepsis.