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To investigate the relation between recreational physical activity and breast cancer, we conducted an analysis of a populationbased casecontrol study of women age 5064 years in western Washington State. The study included 537 patients with breast cancer diagnosed during 19881990 and 492 randomly selected population control women. Detailed information on recreational exercise habits and other risk factors for breast cancer was ascertained by structured inperson interviews. A similar proportion of cases and controls reported exercise in the 2 years before the referent date about 50, or during ages 1221 years about 40. Compared with women who reported no exercise, there was a slightly decreased risk of breast cancer in women who exercised more than 1.5 hours per week in the 2 years before diagnosis cases or referent date controls odds ratio OR 0.7 95 confidence interval CI 0.41.1, or who engaged in at least some highintensity exercise OR 0.7 95 CI 0.41.1, but there was no clear decline in risk at the highest categories of duration or intensity. There was no association between any intensity exercise at ages 1221 years and risk of breast cancer. These results indicate a weak negative association between physical activity and risk of breast cancer in middleaged women.