They found that women aged 50 to 59 who took estrogen were 30 percent to 40 percent less likely than women taking placebos to have large amounts of calcified plaque in their arteries - a widely accepted predictor of heart attack risk.

Dr. JoAnn Manson of Harvard Medical School's Brigham and Women's Hospital, who led the study, said the results should be reassuring to women in their 50s who have been taking estrogen for their menopausal symptoms.

But, she stressed, hormone replacement therapy, or HRT, is not completely safe and women should not take it unless they have a good reason to.

"Hormone therapy should not be used for the express purpose of preventing cardiovascular disease due to other known risks, and it should be limited to treatment of menopausal symptoms at the lowest dose for the shortest duration necessary," Manson said in a telephone interview.

"So I do not believe it should be started in all women for the purpose of protecting their hearts."

Manson led the original Women's Health Initiative study, which found in 2002 that hormone replacement therapy raised the risk of blood clots, breast cancer and heart attacks.

Many women stopped taking HRT, although researchers at the time noted that many women in their study were well past menopause, with an average age of 63.

"There are still many women out there with moderate to severe symptoms who are not seeking treatment because they are concerned that estrogen could increase the risk of heart disease and other health problems," Manson said.

Source: New England Journal of Medicine