A case of hyperammonemia induced by chemotherapy, including high-dose fluorouracil(5-FU), for advanced unresectable large intestinal cancer has been reported. This case involved an 81-year-old female who was diagnosed with pT4bcN2M1 (multiple hepatic metastases; stage Ⅳ; KRAS: mutant)after emergency surgery for sigmoid colon cancer and diffuse peritonitis. Post-operation, the 4 courses of mFOLFOX6 plus Bmab therapy was started for advanced unresectable recurrent large intestinal cancer; 48 hours later, she developed consciousness disorder(JCS Ⅲ-300). The disorder promptly disappeared after discontinuation of high-dose 5-FU. Because high-dose 5-FU was inferred to be the main cause of hyperammonemia, XELOX plus Bmab therapy was started as a post-treatment. She did not develop hyperammonemia; therefore, 8 courses were administered. The patient is being followed-up now.