Factors influencing circulating estrogen levels, insulin-mediated pathways or energy balance through obesity-related mechanisms, such as physical activity, have been proposed as potential risk factors for endometrial cancer. We examined measures of physical activity in relation to endometrial cancer risk in the American Cancer Society Cancer Prevention Study II Nutrition Cohort, a prospective study of cancer incidence and mortality, using information obtained at baseline in 1992. From 1992 to 2003, 466 incident endometrial cancers were identified among 42,672 postmenopausal women with intact uteri who were cancer-free at enrollment. Cox proportional hazards modeling was used to compute hazard rate ratios (RR) while adjusting for potential confounders. To assess the role of body mass index (BMI) in this relationship, we computed multivariate RR with and without adjustment for BMI and stratifying by BMI. All measures of physical activity and the avoidance of sedentary behavior were associated with lower endometrial cancer risk. Baseline recreational physical activity was associated with 33% lower risk (RR = 0.67, 95% CI 0.44-1.03 for 31.5+ vs. <7 MET-hr/week, trend p = 0.007) in the multivariate model without BMI. However, the trend was attenuated after further adjustment for BMI (trend p = 0.18). BMI significantly modified the association between physical activity and endometrial cancer risk (heterogeneity of trends p = 0.01). The inverse relationship was seen only among overweight or obese women (trend p = 0.003) and not in normal weight women (trend p = 0.51). In summary, light and moderate physical activity including daily life activities were associated with lower endometrial cancer risk in our study, especially among women who are overweight or obese.