The postmenopausal state is associated with an increase risk for heart disease. Much of this increase in risk may be due to the loss of estrogen (the main female hormone) and the effect of this loss on lipids (blood fats). This loss of estrogen is often treated by estrogen replacement therapy. Estrogen replacement therapy seems to have a beneficial effect on lipid levels. The purpose of this research study is to understand 1) how menopause affects lipids and 2) how hormone replacement therapy effects the lipid metabolism of postmenopausal women.