BACKGROUND: Most studies on endocrine-disrupting chemicals and breast cancer have focused on single compounds and have produced inconclusive findings. OBJECTIVES: We assessed the combined estrogenic effects of mixtures of xenoestrogens in serum and their relationship to breast cancer risk. METHODS: A total of 186 incident pretreatment breast cancer cases and 196 frequency-matched controls were randomly sampled from a large population-based multicase-control study in Spain. The total effective xenoestrogen burden attributable to organohalogenated xenoestrogens (TEXB-α) and endogenous hormones and more polar xenoestrogens (TEXB-β) was determined in serum samples using high-performance liquid chromatography and E-Screen bioassay. Odds ratios for breast cancer comparing tertiles of serum TEXB-α and TEXB-β were estimated using logistic models, and smooth risk trends were obtained using spline models. RESULTS: Cases had higher geometric mean TEXB-α and TEXB-β levels (8.32 and 9.94 Eeq pM/mL, respectively) than controls (2.99 and 5.96 Eeq pM/mL, respectively). The fully adjusted odds ratios for breast cancer (95% confidence intervals) comparing the second and third tertiles of TEXB-α with the first tertile were 1.77 (0.76, 4.10) and 3.45 (1.50, 7.97), respectively, and those for TEXB-β were 2.35 (1.10, 5.03) and 4.01 (1.88, 8.56), respectively. A steady increase in risk was evident across all detected TEXB-α levels and a sigmoidal trend was observed for TEXB-β. Individual xenoestrogens showed weak and opposing associations with breast cancer risk. CONCLUSIONS: This is the first study to show a strong positive association between serum total xenoestrogen burden and breast cancer risk, highlighting the importance of evaluating xenoestrogen mixtures, rather than single compounds, when studying hormone-related cancers. ; This study was partially supported by Acción Transversal del Cancer, approved by the Spanish Ministry Council on 11 October 2007; Carlos III Institute of Health (grants PI08/1770, PI09/00773- Cantabria, PI11/00610, PI12/00265, PI12/00488, PI12/00715, and PI12/01270); and Fundación Marqués de Valdecilla (grant API 10/09). Biological samples were stored at biobanks supported by the Carlos III Institute of Health and the European Regional Development Fund (MAR Biobank RD09/0076/00036 and ISCIII Biobank RD09/0076/00108).