Background: Increased physical activity has been hypothesized to be a means of breast cancer prevention. We examined the associations between physical activity at two different times in life and breast cancer risk. Methods: We analyzed data from the Nurses' Health Study II, a prospective study of women aged 25-42 years in 1989. On the baseline survey, women were asked, 'While in high school and between the ages 18 and 22 years, how often did you participate in strenuous physical activity at least twice a week?' We averaged answers to these two questions to develop a measure of late adolescent activity. Women were also asked at baseline to report the number of hours per week they currently spent in different nonoccupational activities. During 6 years of follow-up, we identified 372 cases of invasive breast cancer. Data were analyzed by use of multivariate pooled logistic regression to produce relative risk (RR) and confidence intervals (CIs) of being diagnosed with the disease. Results: Women who were more active in late adolescence were not at reduced risk of breast cancer compared with less active women. For those women who reported engaging in strenuous activity at least twice per week for 10-12 months per year in late adolescence, the RR of cancer, compared with those who never engaged in such activity, was 1.1 (95% CI = 0.81.6). Similarly, higher levels of recent nonoccupational physical activity were not associated with reduced risk of breast cancer (RR for ≤7 hours of activity/week relative to <1 hour/week = 1.1; 95% CI = 0.8-1.5). Conclusion: Our findings do not support a link between physical activity, in late adolescence or in the recent past, and breast cancer risk among young adult women.