Endothelial dysfunction is a disordered function of very small coronary vessels that cannot be seen by angiograms done during cardiac catheterization. The impairment of endothelial function is the primary etiology implicated in the origin and development of atherosclerosis. In a prospective study of 2264 post-menopausal women, endothelial function was measured by utilizing the noninvasive measurement of the brachial artery flow characteristics.