Hyperammonemic encephalopathy is a rare adverse event of chemotherapies based on high-dose 5-fluorouracil. We present a woman in her 70s with metastatic pancreatic adenocarcinoma who underwent FOLFIRINOX therapy. She developed acute onset disturbance of consciousness after completing the first 5-fluorouracil infusion cycle (2400 mg/m(2)/46h). We suspected hyperammonemic encephalopathy induced by 5-fluorouracil and administered branched-chain amino acids solutions and she recovered within a few hours of treatment. Brain computed tomography and magnetic resonance imaging revealed no abnormal findings. She subsequently received chemotherapy with gemcitabine and developed no further hyperammonemia. To the best of our knowledge, this is the first report of FOLFIRINOX-induced hyperammonemic encephalopathy in a patient with pancreatic cancer.