Injury or activation of the endothelium changes its regulatory functions and results in abnormal endothelial cell function. Dysfunction of the endothelium has been defined as an imbalance between relaxing and contracting factors, between procoagulant and anticoagulant mediators or between growth-inhibiting and growth-promoting substances. The first part of this review describes endothelial dysfunction in hypercholesterolemia, and atherosclerosis in intimal thickening induced by perivascular and intravascular techniques. We focus on the implications of endothelial dysfunction in these conditions and the role of nitric oxide, endothelium-dependent hyperpolarization, endothelin, cytokines, adhesion molecules, growth factors, and thrombosis. In the second part, the endothial dysfunction in other diseases including hypertension, syndrome X, and diabetes is discussed. Finally, a short overview of therapeutic approaches of the dysfunctional endothelium is given.