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 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.
"Women don't have to run marathons for the maximum benefit," Holmes said.
The cancer society's Saslow cautioned that the findings should not be misinterpreted to mean that just because someone exercises she is guaranteed to survive, or that she should feel guilty or be blamed if she does not exercise.
"Sometimes people think, 'If I take chemotherapy, it's going to save my life and have a huge impact.' In many women, it makes a small impact. This is in line with the difference treatment makes," Saslow said. "And certainly no one is going to blame a woman for what she doesn't do. But if you can do it, this says you might benefit."
Many 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."
