This article reports the surface structure of the endothelial cells covering the cardiac valves and their changes in chronic inactive rheumatic valvulitis and bacterial endocarditis. The endocardial surface of the normal cardiac valve was covered by a layer of endothelial cells, each of which was recognized because of the presence of centrally located nuclear bulge and marginal folds at the cell boundaries. In the cardiac valves affected by chronic rheumatic valvulitis, the endothelial layer denuded extensively, even in the regions where only slight thickening was noticed in the intravalvular tissues. As compared to the inflow side of the valve, denudation of the endothelial layer was more conspicuous on the outflow side. There was no morphological difference between the two hemodynamically different conditions, i.e. stenosis and regurgitation. In the cardiac valves affected by bacterial endocarditis, in contrast, the destruction of the endothelial layer was preferentially observed along the closing margins of the valve, and in the region of calcification and vegetation. These findings are compatible to the interpretation that differences in morphological changes of the endothelial layer in these two acquired valvular diseases may reflect the differences of their pathogeneses. The present study thus suggest an importance of endothelial cell in elucidating the pathogenesis of acquired valvular diseases.