The vascular endothelium importantly modulates many biochemical and physiologic properties of the vascular wall, including the release of potent vasoactive factors, maintenance of an anticoagulant state and modulation of vascular growth by the release of both proliferative and antiproliferative substances. Prominent among these endothelial roles is the production of a potent vasodilator or family of vasodilators termed the endothelium-derived relaxing factor, one of which has been identified to be nitric oxide or a closely related compound. Several diseases that commonly occur in patients undergoing coronary angioplasty alter many facets of endothelial function and may predispose to adverse clinical sequelae of angioplasty, including immediate thrombosis, restenosis and vascular spasm. Several features of the endothelium-derived relaxing factor, how it is altered by pathologic processes and how these considerations relate to the immediate and late postangioplasty periods are discussed.