Considerable epidemiological data have accumulated in support of a lower risk of breast cancer among physically active women. Few studies, however, have examined the relation with lifetime physical activity from all sources, and moderate activity in particular. We conducted a population-based case-control study of breast cancer in Latinas, African Americans, and whites aged 35-79 years to assess the association with lifetime histories of moderate and vigorous physical activity, including recreational activity, walking, bicycling, household and outdoor chores, and occupation. Patients diagnosed with invasive breast cancer between 1995 and 1998 were identified through the cancer registry in the San Francisco Bay area, and a random sample of women without breast cancer was identified through random-digit dialing. A structured questionnaire administered in-person was completed by 403 premenopausal cases and 483 controls and 847 postmenopausal cases and 1065 controls. Summing activities from all sources over an individual's lifetime, we found reduced breast cancer risk in both pre- and postmenopausal women with the highest versus lowest tertile of average lifetime activity (premenopausal: multivariate adjusted odds ratio = 0.74, 95% confidence interval = 0.52-1.05; postmenopausal: odds ratio = 0.81, 95% confidence interval = 0.64-1.02), with similar reductions in the three racial/ethnic groups. In premenopausal women, risk reductions were similar for different types of activities, whereas in postmenopausal women, they were limited to occupational activity. Considering the intensity of activities, risk reductions were similar for moderate and vigorous activities. Because few of the currently known risk factors for breast cancer are modifiable, these results underline the public health importance of promoting physically active lifestyles.