Purpose Increased reactive oxygen species may exhaust the antioxidant capability of human defense systems, leading to oxidative stress and cancer development. Urinary F2-isoprostanes, secondary end products of lipid peroxidation, are more accurate markers of oxidative stress than other available biomarkers. No prospective study has investigated whether levels of 15-F_2t-isoprostane (15-F_2t-IsoP) and its metabolite 2,3-dinor-5,6-dihydro-15-F_2t-IsoP (15-F_2t-IsoPM) are related to breast cancer risk. Patients and Methods We conducted a nested case-control study within the Shanghai Women's Health Study, a population-based cohort study of 74,942 Chinese women between 40 and 70 years of age. Prediagnostic urinary 15-F_2t-IsoP and 15-F_2t-IsoPM were measured by gas chromatography mass spectrometry for 436 breast cancer cases and 852 individually matched controls. Results Urinary excretion of isoprostanes was not significantly different between cases and controls. However, among overweight women, levels of isoprostanes were positively associated with breast cancer risk, which became stronger with increasing body mass index (BMI). Among women with a BMI ≥ 29, the odds ratio (OR) increased to 10.27 (95% CI, 2.41 to 43.80) for the highest compared with the lowest tertile of 15-F_2t-IsoPM (P for trend = .003; P for interaction = .0004). In contrast, 15-F_2t-IsoP and 15-F_2t-IsoPM were inversely associated with breast cancer risk among nonoverweight women. Among women with a BMI ≤ 23, breast cancer risk was reduced with increasing 15-F_2t-IsoP levels in a dose-response manner (P for trend = .006), with an OR of 0.46 (95% CI, 0.26 to 0.80) for the highest tertile versus the lowest (P for interaction = .006). Conclusion Our results suggest that the role of oxidative stress in breast cancer development may depend on adiposity.