 The primary hypothesis to be tested is that 17B-estradiol estrogen will reduce the progression of early atherosclerosis if initiated soon after menopause when the vascular endothelium lining of blood vessels is relatively healthy versus later when the endothelium has lost its responsiveness to estrogen. Ultrasonography will be used to measure the rate of change in the thickness of the carotid artery and cardiac computed tomography CT will be used to measure coronary artery calcium and coronary artery lesions. The second hypothesis to be tested is that 17B-estradiol estrogen will reduce the progression of cognitive decline if initiated soon after menopause when healthy brain tissue remains responsive to estrogen versus later when brain tissue has lost its responsiveness to estrogen. 