['To examine the relationship between physical activity and breast cancer in African American women.', 'A population-based case-control study was conducted with 199 women (97 cases and 102 controls) from the Washington, DC metro area. A physical activity questionnaire elicited responses on frequency of walking for exercise and vigorous physical activity (eg, running, aerobics) in the past year. Responses were used to calculate a metabolic equivalent (MET) score (MET-hours/week = hours/week vigorous activity x 7 + hours/week walking x 3). The MET score was categorized into low, medium, and high tertiles. Multivariate logistic regression examined the association between physical activity and breast cancer.', 'African American women who engaged in vigorous physical activity (> or = 2 hours/week in the past year) had a 64% reduced risk of breast cancer compared to those who did not participate in any vigorous activity (odds ratio, OR = .36; 95% confidence interval, CI = .17-.75). We also found a 64% reduced breast cancer risk in women with a high vs low tertile of total activity (OR = .36; 95% CI = .16-.79). For postmenopausal women, vigorous physical activity and total activity (high vs low tertile) also had an inverse relationship with breast cancer (P<.05).'] Does physical activity reduce breast cancer risk in African American women?