We report two patients having hyperammonemic encephalopathy while being treated with chemotherapy for colorectal cancer. The first patient was a 69-year-old man with sigmoid colon cancer, having a massive invasion to the urinary bladder. He received SOX therapy following a pelvic exenteration operation. After the third course of SOX therapy, he presented with general fatigue and repeated seizures, and blood examination showed a high level of serum ammonium. He was diagnosed as hyperammonemic encephalopathy. The second patients was a 60-year-old woman with ascending colon cancer and liver metastasis having portal vein tumor thrombosis, who was given a palliative resection of ascending colon, and then underwent modified FOLFOX6 therapy. At the second course, she fell into a deep coma, and blood examination revealed a high level of serum ammonium. In both patients, treatment with infusion of branched-chain amino acid solutions resolved the symptoms of encephalopathy. Acute neurotoxicity caused by hyperammonemic encephalopathy during chemotherapy for colorectal cancer is rare and not well recognized, but it is a clinically important complication. We should pay more attention to hyperammonemic encephalopathy of patients receiving chemotherapy for colorectal cancer.