Abstract Trans fatty acids (TFAs) is a recent focus of health problems. TFA is a definitive risk factor for cardiovascular diseases and the death. TFA is also possible risk factor for Alzheimer's disease, obesity, diabetes, fatty liver, and ovulation infertility. Relation of TFA with carcinogenic risk is controversy; however, TFA is reported to increase the risk of breast cancer and prostate cancer. Elaidic acid (EA), a trans form of oleic acid, enhances cancer cell growth, invasion, and anti-apoptotic survival. By animal models, EA promotes tumor growth and metastasis to the lung, liver, and peritoneum. EA induces stemness in cancer cells through transactivation of EFGR via SRC from GPR40/120 as receptors in EA-integrated cholesterol raft. Activated EGFR relays the signals to activate canonical and non-canonical wnt pathway and to inactivate notch1 pathways. EA also increases miR-494, which inhibits cell differentiation through decrease of target genes. Continuous EA feeding with dosage alteration increased cancer cell stemness. EA diminishes the efficiency of 5-fluorouracil by increase of residual cancer stem cells. These findings suggest that TFA is a relevant cancer promoting factor. Decision of removal of TFA from foods by FDA might provide an impact to cancer clinics. Citation Format: Hitoshi Ohmori, Toshio Kuwai, Yasuhiko Kitadai, Kiyomu Fujii, Rina Fujiwara, Yukiko Nishiguchi, Tomonori Sasahira, Hiroki Kuniyasu. Protumoral role of trans fatty acid in colorectal cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1699.