The authors made a qualitative cytological analysis of the percentage of the heterogenic endothelium and its association with the severity of atherosclerosis. A total of 15,000 endotheliocytes (usual-type, giant uninucleated, binucleated, trinucleated, multinucleated, and horseshoe-shaped nuclei) were studied above different forms of atherosclerotic lesion of the human aorta and pulmonary artery. The material was obtained from 5 deceased patients aged 35 to 85 years. Both uninucleated and multinucleated cells were identified as the endothelium by an immunohistochemical test in the detection of specific factor VIII antigen in their cytoplasm. The proportion of heterogenic cells above the intact portions of the pulmonary artery and aorta was ascertained to be 13.7 and 24.8%, respectively; at the same time that of this endothelium above the fatty streaks increased up to 56.1 and 58.7%, respectively; and that above fatty plaques did up to 72.2 and 73.1%, respectively. Thus, the total number of endotheliocytes with the usual type of nuclei statistically significantly decreased above atherosclerotic lesions (fatty streaks and plaques) of the pulmonary artery and aorta to 35.8 and 31.8%, respectively. Extrusion of a population of cells with the usual type of nuclei with the progress of atherosclerotic lesions is attended by an increase in the proportion of the heterogenic endothelium up to 64.2 and 68.2%, respectively. It is suggested that the optimum combination of variable dimensions of the endothelium and the size and number of their nuclei is an adaptive reaction and an essential prerequisite to the most effective endothelial function when atherosclerotic lesion of the arterial intima progresses.