Abstract Successful treatment of triple negative (ER-, PR-, HER2-) breast cancers (TNBC) remains elusive because of the limitations of currently available chemotherapeutic agents for TNBC, drug resistance and drug toxicities. Docosahexaenoic acid (DHA), an n-3 polyunsaturated fatty acid, has been shown to exhibit anticancer actions both in vitro and in vivo in a variety of cancers, including human breast cancer. Here, we report that DHA induces apoptosis in TNBC cell lines, which is associated with cleavage of caspase-8 and -9. DHA induces endoplasmic reticulum (ER) stress evidenced by the increased levels of ER stress markers, GRP78, CHOP, and spliced XBP-1, and induced increased levels of death receptor DR5 protein expression. siRNAs to CHOP and DR5 blocked DHA-induced apoptosis and siRNA to CHOP blocked DHA-induced upregulation of DR5, indicating that DHA induces ER stress mediated death receptor dependent apoptosis in TNBC cells via CHOP. Furthermore, our data show that DHA mediated increases in levels of reactive oxygen species (ROS), apoptosis and ER stress can be blocked by antioxidants; N-Acetyl Cysteine (NAC) and alpha-tocopherol (αT) indicating that ROS plays a critical role in DHA-induced apoptosis and ER stress. In contrast to αT, gamma-tocotrienol (γT3), another form of vitamin E, cooperated with DHA to induce apoptosis and ER stress in TNBC cells. siRNAs to CHOP and DR5 blocked the combination effects mediated by DHA + γT3 on induction of apoptosis and ER stress. Taken together, our data, for the first time, demonstrate that DHA induces apoptosis in TNBC cells via activation of an ER stress mediated DR5 dependent pro-apoptotic pathway. Our data also show that different forms of vitamin E exhibit different combinational effects on DHA-induced apoptosis; namely, blockage by αT and enhancement by γT3. (Funding for these studies was provided by the Clayton Foundation for Research). Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4953. doi:1538-7445.AM2012-4953