Abstract Background: The rising incidence of breast cancer is mainly due to changes in reproductive, lifestyle, and environmental factors, not inherited genetic mutations. Many risk factors can be modified, offering important opportunities for prevention. In the era of personalized care, treatment is subtype dependent. While most prevention strategies are not subtype specific, we wanted to see if breastfeeding confers the same protection based on subtype, especially against poorer prognostic subtypes. Methodology: Relevant articles from case-control or prospective studies were identified by searching PubMed and Scopus databases through 2013 and reference lists of relevant articles. Two researchers independently did the search and evaluated the articles. The summary risk estimates and 95% confidence intervals were calculated using random effects models (DerSimonian-Laird method) for the association between breastfeeding and breast cancer by receptor status. The reference category in most of the studies was never breastfeeding, but in a few studies this also included women who breastfed for a short time. Results: This meta-analysis of 26 articles (20 from case-control, 6 from prospective studies) published between 1983 and 2013 included 34,479 women with invasive breast cancer from 8 countries. Breastfeeding was inversely associated with breast cancer risk across all combination of subtypes: ER+ or -, PR+ or -, HER2+ or -. The relative risk reduction ranged from 12–29% when only the results adjusted for age, body mass index (BMI), parity, and family history of breast cancer were included. There did not appear to be a differential protective benefit of breastfeeding based on subtype using ever vs never analysis. (A breastfeeding dose response analysis by subtype is in progress.) Association between ever breastfeeding and breast cancer risk by receptor statusReceptor statusNo of studiesOR (95% CI)I2 statistics (%)P for heterogeneityNon–luminal (ER–, PR–)Cohort70.84 (0.72 – 0.97)500.06Cohort, adjusted*30.88 (0.74 – 1.06)420.18Case–control130.76 (0.67 – 0.86)590.004Triple–negativeCohort30.74 (0.62 – 0.88)00.46Cohort, adjusted*10.81 (0.62 – 1.04)––Case–control80.73 (0.64 – 0.84)120.34Luminal (ER+ and/or PR+)Cohort80.98 (0.89 – 1.07)75<0.001Cohort, adjusted*31.04 (0.98 – 1.10)00.75Case–control180.82 (0.76 – 0.89)69<0.001–, negative; +, positive; ER, estrogen receptor; PR, progesterone receptorI2 statistics show % of total variation across studies that is due to heterogeneity rather than random variation (chance). Arbitrarily, I2 percentages may be interpreted as follows: ∼25%, low; ∼50%, moderate; and ∼75, high heterogeneity.*Results adjusted at least for age, body mass index, parity, and family history of breast cancer. Conclusion: Breastfeeding is a powerful strategy to reduce breast cancer risk, with a relative risk reduction of 12–29%. This benefit is larger than previously reported and was independent of hormone- and HER2-receptor status, suggesting mechanisms of action independent from these receptors and their ligands. To maximize breastfeeding use for the long-term health of mothers and babies, it is important to remove barriers in the home, community, and workplace as well as provide education and support before and after delivery. Citation Format: Marisa Weiss, Ying Liu, Paolo Boffetta, Graham Colditz, Ahmedin Jemel, Farhad Islami. Association between breastfeeding and breast cancer risk by receptor status: A meta-analysis [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P5-13-01.