A 70-year-old man was diagnosed with colon cancer with multiple liver metastases.He was administered modified FOLFOX6 plus panitumumab as first-line chemotherapy.He showed consciousness disturbance on the 3rd day during the 8 cycle and was hospitalized urgently.We diagnosed him with 5-FU-induced hyperammonemia.Administration of branchedchain amino acid preparation improved his consciousness disturbance.After changing the regimen of chemotherapy to another one containing oral fluoropyrimidine, the recurrence of hyperammonemic encephalopathy was not found.