Key discoveries in the past decade have revealed that the vascular endothelium is an important regulatory organ that is involved in maintaining cardiovascular homeostasis in health and contributes significantly to the pathomechanism of several cardiovascular diseases. Occupying a strategically important location between circulating blood and tissues and having the ability to respond to changes in its physical, chemical, and humoral environment by the production of a host of biologically active substances, the normal endothelium modulates the tone of underlying vascular smooth muscle, maintains a nonadhesive luminal surface, and mediates hemostasis, cellular proliferation, and inflammatory and immune mechanisms in the vascular wall. Modulation of smooth-muscle tone is mediated by the synthesis release of endothelium-derived relaxing [PGI2, EDRF(NO), and EDHF] and contracting factors (arachidonic acid metabolites, endothelin-1, and angiotensin II). Anticoagulant, fibrinolytic, and antithrombotic properties contribute to the maintenance of the fluidity of blood. Injury or activation (by cytokines) of endothelial cells disrupts these normal regulatory mechanisms and results in morphologic and functional alterations (phenotypic changes) commonly defined as endothelial dysfunction. Clinically, the "syndrome" of endothelial cell dysfunction can be described as generalized or localized vasospasm, thrombosis, atherosclerosis, and restenosis. Although its importance is clearly established, no drugs used today were originally targeted for the treatment of endothelial dysfunction. Recent studies, however, showed that some existing therapies (e.g., angiotensin-converting enzyme inhibitors) may protect the endothelium. Novel diagnostic techniques and innovative therapeutic strategies, based on the already known molecular mechanisms of endothelial dysfunction, are briefly outlined. Further knowledge of the pathobiology of the impaired endothelium will contribute to unraveling some of the remaining mysteries of many cardiovascular diseases and will enable us to design novel therapies to prevent and treat them.