Chemotherapy with 5-fluorouracil (5 FU) has been widely used to treat advanced gastric cancer. Knowing the side effects is therefore important in order to better prevent them. Fluoropyrimidine-induced hyperammonemic encephalopathy is a rare complication and characterized neurological status with elevated ammonia level without radiological abnormalities. We report the first case of 5 FU-induced hyperammonemic encephalopathy in women patients on induction chemotherapy for gastric cancer in Madagascar. His ammonia level (NH 3 ) was 102 µmol/l. The patient recovered from his confusional state after two days of treatment with hyperhydration and vitamin therapy.