Hyperammonemia induced by 5-fluorouracil(5-FU)is known as a rare adverse event, but there are few reports of hyperammonemia occurring during FP(5-FU plus CDDP)treatment for esophageal cancer. We report a case of esophageal cancer with consciousness disorder due to hyperammonemia during FP treatment with an examination of some of the relevant literature. The patient was a man of approximately 70 years of age who was received FP treatment. He showed consciousness disorder on day 4. A blood test showed hyperammonemia(427μg/dL), which was considered to be the cause of his consciousness disorder. He was treated with branched chain amino acid infusion, lactulose and kanamycin and made a full recovery. An operation for esophageal cancer was performed after 3 months and he is currently followed up without recurrence. Hyperammonemia should be considered as a differential diagnosis of consciousness disorder during chemotherapy including 5-FU.