ATHEROSCLEROTIC lesions containing intimal deposits of lipid or calcium occur characteristically only in those portions of the vascular system in which the blood pressure ordinarily is relatively high. Fibrous intimal thickenings sometimes found in sclerotic veins rarely contain either of these materials. Intimal lipid deposits are seldom found in the pulmonary arteries or in very small systemic arteries unless the blood pressure in these areas has been abnormally high. Lesions analogous to atheromatous plaques of arteries may form in the endocardium but only in those portions in which a high level of intracardiac pressure normally is developed, for example, in the aortic valve or the ventricular aspect of the anterior mitral valve leaflet. In coarctation of the isthmus of the aorta, atheromatous plaques are common proximal to and uncommon distal to the point of narrowing. These well established observations suggest that a threshold of hydrostatic pressure may exist below which