A 14-month-old child visited emergency room with stuporous mental state. He had been suffering from failure to thrive (FTT) and emaciation since three months of age, but he had good appetite and had been euphoric. A large mass was found in the third ventricle by brain CT and MRI. His parents refused operative removal of the mass and he expired 14 days after admission. It is necessary to include diencephalic syndrome in the differential diagnosis of failure to thrive when infants show FTT despite of good appetite and euphoric status.