The increasingly common practice of removing both breasts while they are healthy is an effective, if radical, way of preventing breast cancer in women at high risk for the disease, researchers say.

The findings published in today's New England Journal of Medicine were first reported by The Associated Press in 1997 when they were released at a medical conference in San Diego.

The researchers said they consider prophylactic mastectomy, as it is called, to be at least 90 percent effective in reducing breast cancer. It's not totally effective because the disease may already have spread before the breasts are removed.

Dr. Lynn Hartmann and colleagues from the Mayo Clinic reviewed the cases of 639 women who had both breasts removed because of their family history of breast cancer.

Among the 425 considered to be at moderate risk, 37 cases of breast cancer would have been expected after 14 years of follow-up. Only four actually occurred.

The doctors also followed the sisters of the 214 considered at high risk. None of these sisters got prophylactic mastectomies, and 39 percent developed breast cancer. By comparison, just over 1 percent of the high-risk women getting mastectomies later had breast cancer.

``This can only be viewed as good news for women who are considering this option and even better news for women who have already undergone the procedure,'' Drs. Andrea Eisen and Barbara Weber wrote in an accompanying editorial.