For decades, observers have noted that women lose their natural resistance to coronary disease following menopause. It is now known that this increase in coronary risk is due to the loss of the protective effect of estrogen. Although still somewhat controversial, estrogen replacement therapy appears to offer significant cardioprotective benefits to postmenopausal women. These benefits extend to women with a history of coronary artery disease as well as to those without. The exact mechanism by which estrogen confers protection is not known, although the primary focus has been on estrogen's effect on serum lipids. Estrogen reduces total cholesterol levels and low-density-lipoprotein cholesterol levels and increases levels of high-density lipoproteins. Recent data suggest that estrogen may also have a beneficial effect on lipoprotein (a), a less studied lipid fraction, and may affect lipid metabolism in the blood-vessel wall. Numerous studies are under way to define better the risks and benefits of estrogen replacement therapy. Until more is known, the decision to recommend estrogen replacement therapy must be made on an individual basis.