Systemic chemotherapy based on 5-fluorouracil(5-FU)is a standard treatment for unresectable or recurrent colon cancers. Here, we report a case of hyperammonemia induced by chemotherapy using 5-FU for metastatic colon cancer. An 84-yearold male patient with past histories of liver cirrhosis related to hepatitis C virus and renal dysfunction underwent an operation for the rectosigmoid colon cancer 8 years ago. Three years after that operation, a local recurrence of the colon cancer was diagnosed, and chemotherapy using sLV5FU2 was initiated. The recurrence lesion reduced markedly by this chemotherapy, which was ceased 2 years ago. Two years after the cessation the recurrent tumor had been enlarged, and the chemotherapy using the same drugs at the same dose was performed. On the treatment day 3, he was emergently transported to our hospital due to the disturbance of consciousness. Since laboratory tests showed the high concentration of plasma ammonia and the progressed renal dysfunction with no other definite cause of obnubilation, he was diagnosed as the hyperammonemia induced by 5-FU. He was treated by administrating the branched-chain amino acids solutions combined with fluid therapy, which quickly recovered him from the encephalopathy. He is followed up without any chemotherapy because of his high age. When a patient treated with the chemotherapy using 5-FU is sent with the disturbance of consciousness, we should take hyperammonemia into consideration, especially when he or she has the hepatic or renal dysfunction.