Reporting in the Feb. 26 issue of the Archives of Internal Medicine, researchers at the University of Southern California, Los Angeles, studied more than 110,000 women, aged 20 to 79, who took part in the California Teachers Study.

At the start of the study, the women were asked about their participation in moderate exercise (such as brisk walking, golf or volleyball) and strenuous exercise (such as swimming laps, aerobics, and running) from high school up until their current age and within the previous three years.

The study authors also collected information about the women's breast cancer risk factors, including race, family history and use of hormone replacement therapy.

Between the start of the study in 1995-96 and 2002, 2,649 of the women were diagnosed with invasive breast cancer and 593 were diagnosed with in situ (noninvasive) breast cancer.

Women with a long-term history of doing more than five hours per week of strenuous exercise were 20 percent less likely to develop invasive breast cancer and 31 percent less likely to develop in situ breast cancer than those who did less than 30 minutes of strenuous exercise per week, the researchers found.

Strenuous exercise within the previous three years did not offer the same benefit, they noted.

Long-term or more recent moderate exercise also did not appear to affect the risk for invasive or in situ breast cancer, the study found.

The researchers also found that strenuous exercise affected the risk of different types of cancer in different ways. Specifically, strenuous activity may be associated with a lower risk of estrogen-receptor-negative breast cancers, but not estrogen-receptor-positive breast cancers.

"In summary, these results provide additional evidence supporting a protective role for long-term strenuous recreational physical activity on risk of invasive and in situ breast cancer, whereas the beneficial effects of moderate activity are less clear," the authors wrote.