Many previous studies have suggested hormone replacement therapy decreases the risk of heart disease in postmenopausal women because it improves cholesterol levels and appears to keep arteries and vessels in good tone. But no study to date has proven it, even though the estrogen pill Premarin is one of the most widely prescribed medicines in the country.

In one arm of the ongoing federally funded Women's Health Initiative, more than 27,000 women ages 50-79 are taking either estrogen with progestin, estrogen alone or a placebo, in part to test its effect on heart disease. After two years, researchers noted a small-less than 1 percent-increase in cardiovascular "events" among women taking a hormone or hormones compared with those on a placebo.

Dr. Jacques Rossouw, acting director of the Women's Health Initiative, said the increase appears to have been short-lived and has disappeared over time: Two-thirds of the women in the study have been on hormones or a placebo for more than two years and one-third have been on them three years or more.