A study found that hormone therapy does not lower heart risks among all women

(CNN) -- Doctors have been prescribing hormone replacement therapy for women after menopause for years, in part because it was believed to help protect them from heart disease.

But a new, landmark study has shown that hormone replacement therapy does not protect the heart and may actually prove harmful to women with heart disease.

Researchers emphasized that the study results do not apply to healthy women.

The Heart and Estrogen/Progestin Replacement Study, known as HERS, followed 2,700 older women with heart disease for four years.

It found no difference in the rate of heart attacks, or heart attack deaths, between the women who did, and did not, take hormone therapy. It did, however, show an increased risk in heart disease during the first eight months of hormone use.

"We were very surprised by this finding," said Dr. Trudy Bush, a researcher with the University of Maryland. "I think no one expected we would see this early increased risk in heart disease."

While the study linked a rise in heart disease with short-term hormone use, it showed that over the long-term of three to four years, hormone therapy benefits heart disease patients.

So what does this mean for menopausal women with established heart disease?

"At this time, we don't recommend that women with heart disease start hormone therapy if they haven't been on it," Bush said. "But we do recommend that women with heart disease that have been on hormone therapy should stay on it."

CNN Medical Correspondent Rhonda Rowland contributed to this report.