U.S. government researchers abruptly ended a clinical trial of hormone replacement therapy involving thousands of women, saying it significantly increased the risk of developing heart disease and breast cancer.

Millions of women around the world use a combination of estrogen and progestin to relieve menopausal symptoms, such as hot flashes and night sweats. But until now, no one has known what kind of long-term health impact replacement therapy might have.

On Tuesday, the National Institutes of Health halted America's largest study of combination hormone replacement therapy involving 16,000 women.

The five-year study, published this week in the Journal of the American Medical Association, concluded that combination hormone replacement therapy increases a healthy woman's risk of breast cancer.

Dr. Jacques Rossouw is acting director of the National Institute of Health's Women's Health Initiative, which sponsored the study. "The decision to stop early was made because there was an increased risk of breast cancer. This was some 26 percent increase in the treatment group versus the placebo group," he said.

Women in the study who took combination estrogen and progestin had a 29 percent higher risk of heart disease compared to women who took dummy pills, and a 41 percent higher risk of stroke.

A trial involving the hormone estrogen will continue for women whose uteruses have been removed. Studies show estrogen alone causes cancer of the uterine lining, which is why it has not been given to menopausal women with intact wombs.

Doctors have long believed that combination hormone therapy helps prevent heart disease and other ailments, something that has now been disproved, according to Dr. Rossouw.

"The bottom line is long-term treatment with estrogen plus protestin to prevent chronic diseases is not a viable proposition. By long-term I mean more than four or five years. Specifically, if should not be used for treating coronary artery disease or strokes. And even for osteoporosis prevention physicians should consider alternatives," Dr. Rossouw said.

The authors of the study stress that the relative risk hormone replacement therapy to women is small, saying for every 10,000 who take the estrogen-progestin combination there will be eight new breast cancers.

They advise women to talk to their doctors about the best course of action for the short term.