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DORN, J., J. VENA, J. BRASURE, J. FREUDENHEIM, and S. GRAHAM. Lifetime Physical Activity and Breast Cancer Risk in Pre- and Postmenopausal Women. Med. Sci. Sports Exerc., Vol. 35, No. 2, pp. 278–285, 2003.This research examined associations between leisure time and occupational physical activity (PA) across the lifespan and pre- and postmenopausal breast cancer. In a population-based case-control study, 301 premenopausal cases, 316 premenopausal controls, 439 postmenopausal cases, and 494 postmenopausal controls, 40- to 85-yr-old reported time spent in exercise or sports strenuous enough to sweat and miles walked per week for time periods 2, 10, and 20 yr before the interview and at age 16. Lifetime occupational history was obtained. Jobs were coded according to the National Cancer Institute’s PA job matrix.Strenuous PA was generally associated with a reduced breast cancer risk. Among women categorized as active at all four periods [at least 91+ h·yr−1 (1.75+ h·wk−1 avg)], a strong, significant protective effect was observed in postmenopausal [odds ratio (OR) 0.50 (0.28–0.90)] but not in premenopausal women [OR 1.06 (0.54–2.08)]. A strong protective effect was observed for activity performed 20 yr prior, in both pre- and postmenopausal women, although CIs overlapped for different time periods. Using women who reported no strenuous activity as the referent, OR (95% CIs) for the highest PA category [182+ h·yr−1 (3.5 h·wk−1 avg)] 20 yr ago were 0.57(0.31-1.05) and 0.51(0.31-0.83) for pre- and postmenopausal women, respectively. Walking was generally unrelated to risk. There was some indication of increased risk for the upper category of occupational PA for postmenopausal women, perhaps related to other industrial occupational exposures.Our results suggest a modest protective effect of strenuous leisure time PA on breast cancer risk in both pre- and postmenopausal women. The effects appear strongest for those active at least 20 yr prior and among postmenopausal women who were consistently active throughout their lifetime.