This histological study of endocardial thickening in human hearts revealed that as in adult hearts, the proliferation in fetal, neonatal, and infant hearts consisted of collagen, elastin, and smooth muscle cells. Variation in severity from chamber to chamber and site to site indicated that severity is not an aging phenomenon and that predominantly local blood flow conditions determine localization and progression of proliferation. The similarity to endocardial thickening of cardiac valves and to intimal proliferation in blood vessels was remarkable. In old age and in chronic rheumatic heart disease the proliferation exhibited hyalinization, cell depletion, loss and fragmentation of elastin, lipid accumulation, and thrombosis, indicative of a similar pathogenesis to atherosclerotic changes in valvular endocardium and blood vessels. It was concluded that these chronic hemodynamically induced degenerative changes in the endocardium, including cardiac valves, should be classified as endocardial atherosclerosis analogous to that in arteries and veins and that severity is aggravated by high blood pressure, cardiac malformations, and dysfunction or damage caused by other disease processes.