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OBJECTIVES: Some studies of white women suggest that exercise reduces the incidence of breast cancer. There are no data on black women. We assessed the relationship between strenuous physical activity and prevalent breast cancer among participants in the Black Women's Health Study. METHODS: Data on strenuous recreational physical activity at various ages and other factors were collected in 1995 by mail questionnaire from 64,524 United States black women aged 21 to 69 years. The 704 women who reported breast cancer (cases) were matched on age and on menopausal status at the time of the breast cancer diagnosis with 1408 women who did not report breast cancer (controls). Odds ratios for levels of physical activity at various ages were derived from conditional logistic regression with control for potential confounding factors. RESULTS: Odds ratios for > or =7 h per week relative to < 1 were significantly reduced for strenuous activity at age 21 for breast cancer overall and premenopausal breast cancer, at age 30 for breast cancer overall, and at age 40 for postmenopausal breast cancer. There was no evidence of a reduction associated with exercise in high school. CONCLUSIONS: The findings of the present study suggest that strenuous physical activity in early adulthood is associated with a reduced risk of breast cancer in African-American women.