Being physically active boosts the odds that breast-cancer patients will survive the disease, according to the first study to produce evidence that exercise improves the prospects of beating any malignancy.

The findings, from a large, well-respected study of U.S. nurses, found that breast-cancer patients who walk or do other kinds of moderate exercise for three to five hours a week are about 50 percent less likely to die from the disease than sedentary women.

The findings add strong support to the growing body of evidence that healthy lifestyle factors such as eating well and exercising regularly provide significant health benefits — possibly even offering protection against cancer recurrences about on par with chemotherapy and even the newer hormonal and drug treatments.

While physical activity is no substitute for medical treatment — and often difficult for exhausted cancer patients — the researchers who conducted the study said the findings indicate breast-cancer patients should try to exercise regularly after undergoing standard care to maximize their chances of surviving.

"Women with breast cancer have little to lose and much to gain from exercise," said Dr. Michelle Holmes of the Harvard Medical School in Boston, who led the study published in today's issue of the Journal of the American Medical Association. "This is good news for women with breast cancer."

The findings are particularly striking because the benefit appeared strongest for the most common form of breast cancer, and held true regardless of whether women were diagnosed early or not until after their cancer had spread.

"This is exciting," said Debbie Saslow of the American Cancer Society. "Women are always asking: 'What can I do?' This is saying: 'There's something women can do that doesn't involve drugs and side effects.' "

Previous research has shown regular exercise reduces the chances of developing many diseases, among them heart disease and various forms of cancer, including breast cancer. Other studies have shown that exercise boosts breast-cancer patients' sense of well-being and quality of life.

The new study is the first, however, to show that regular exercise reduces the death rate among women who have had breast cancer, which hits about 211,000 U.S. women and kills about 40,000 each year, making it the most common cancer and second biggest cancer killer, after lung cancer, among women. It comes on the heels of a study last week that for the first time indicated that low-fat diets can reduce the risk of breast-cancer recurrences.

Holmes and her colleagues examined data on 2,987 women who were diagnosed with breast cancer between 1984 and 1998 and then provided detailed information about how much exercise they routinely got as part of the Nurses' Health Study.

After following the women through 2002, the researchers found that any amount of exercise, even walking just one hour a week, increased the odds of surviving. The degree of protection increased with the amount of activity up to about three to five hours per week.

Of the 959 women who got the least exercise — less than three hours a week — 110 died of breast cancer. In comparison, of the 335 women who got three to five hours of exercise a week, only 20 died of the disease.

The findings held even after the researchers accounted for other factors that could confuse the analysis, such as weight, smoking and eating habits.

The benefit was greatest for women whose breast cancer was sensitive to the hormone estrogen, which is the most common form. Previous research had shown that exercise lowers the levels of estrogen, which can fuel the growth of breast-cancer cells.

"The most logical explanation is that physical activity lowers hormone levels, and the lower hormone levels reduce the chances of a recurrence," Holmes said.

The results indicate that breast-cancer patients who follow current government guidelines for physical activity could significantly boost their chances of surviving.

Many breast-cancer patients end up exercising less because they are worn out by the treatment, noted Anne McTiernan of the Fred Hutchinson Cancer Research Center in Seattle.

"This tells us that women should try to avoid that and really try to exercise after their treatment if at all possible," she said. "A few hours of brisk walking will do."'