Vigorous exercise seems to protect against the development of breast cancer in normal-weight postmenopausal women, US research shows.
Regular activity such as running, aerobics or even heavy housework was associated with a 30% reduced risk of developing the disease, a study found.
Strenuous activity was only protective in women who were not overweight or obese and light exercise had no effect.
The 11-year study of 32,000 women is published in Breast Cancer Research.
Women were asked to fill in a detailed questionnaire about how much and what physical activity they did.
Vigorous exercise was judged to include the more strenuous household and gardening tasks, such as scrubbing floors, washing windows, digging, or chopping wood and sports or exercise such as running, fast jogging, competitive tennis, aerobics, bicycling on hills, and fast dancing.
The figures initially showed a small, non-significant reduced risk of breast cancer associated with vigorous physical activity, the researchers from the US National Cancer Institute reported.
But when they looked only at women of a healthy weight, the association became much stronger.
There was no link with exercise in overweight or obese women.
And there was no difference in the strength of the association when the team looked at whether women had a hormone-dependent type of cancer or not.
Non-vigorous activity, which included light housework such as vacuuming, washing clothes, painting, general gardening and light sports or exercise, such as walking, hiking, light jogging, recreational tennis, or bowling was not protective.
It is not clear exactly why exercise may protect against breast cancer, the researchers said.
Study leader Michael Leitzmann said: "In this cohort of postmenopausal women, breast cancer risk reduction appeared to be limited to vigorous forms of activity.
"It was apparent among normal weight women but not overweight women.
"Our findings suggest that physical activity acts through underlying biological mechanisms that are independent of body weight control."
Henry Scowcroft, science information manager at Cancer Research UK, said: "This study adds to the growing body of evidence that breast cancer is less common amongst post-menopausal women who lead an active lifestyle.
"Although this paper suggests that vigorous activity gives the most benefit, several other studies have shown that less intense activity can also be beneficial in the long run.
"Clearly, we need to do more work to find out exactly how exercise affects breast cancer risk, so that women can make informed decisions about their lifestyle."