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Introduction: Identifying biomarkers of breast cancer risk among young women would have value in developing effective screening and prevention strategies at early ages. We have proposed that DNA methylation analysis of breast milk may provide breast cancer risk information among young women, and could possibly provide etiologic clues related to the higher rates of early onset cancers among African American as compared with White women in the US. Objective: The purpose of this project was to identify associations between genome-wide DNA methylation levels in breast milk and race adjusted for other breast cancer risk factors. Study Population: Cancer-free, lactating U.S. Black (n=57) and White (n=82) women, ages 19 to 44, provided frozen breast milk samples, as well as demographic, behavioral, and reproductive data, to the Breastmilk Laboratory at University of Massachusetts Amherst. Women were uniparous and did not have a personal history of breast cancer at the time of milk donation. Methods: DNA was extracted from breast milk samples using the phenol-chloroform method. Genome-wide methylation analysis was performed on breast milk samples using the Infinium HumanMethylation450 BeadChip. Probes with 50% or more missing data, cross-reactive probes, as well as probes with minor allelic frequency greater than 0.05 in European- or African-Americans were removed, leaving 379,042 CpG sites for analysis. Multivariate generalized linear regression models were used to examine associations between race and other breast cancer risk factors and methylation beta values, adjusting for potential confounding factors. P-values less than 1E-7 were considered statistically significant. Results: Black women in this study were more likely to be never smokers, to not have used over-the-counter pain medication in the past week, and to breastfeed longer. After adjustment by age, BMI, smoking status, and batch number, race was significantly associated with differential methylation at 1143 CpG sites, including 1024 at which Black women demonstrated increased methylation levels. Additionally, breastfeeding duration was associated with 269 CpG sites, with 268 showing a significant inverse relationship with methylation. Methylation sites significantly associated with Black race and lactation duration were located within tumor suppressor and promoter genes as well as in genes implicated in obesity and diabetes. Conclusion: This preliminarily analysis of DNA methylation in breast milk suggests that Black women have increased methylation and longer breastfeeding is associated with reduced methylation. Further research to understand how etiologic factors related to breast cancer may alter DNA methylation patterns in normal breast may lead to improved understanding of breast cancer risk at a young age and potentially causes of racial disparities in breast cancer incidence between White and Black women. Citation Format: Davis Lynn BC, Bodelon C, Pfeiffer RM, Yang HP, Yang H, Lee M, Laird PW, Campan M, Weisenberger DJ, Murphy J, Sherman ME, Browne EP, Anderton DL, Arcaro KF, Gierach GL. Differences in genome-wide DNA methylation levels in breast milk by race and lactation duration [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P6-10-07.