Viral encephalitis can lead to serious neurological sequelae and death among younger children. It is also known that the mortality rate in encephalitis with cerebral edema or transtentorial brain herniation is higher. A 4-year-old boy visited our emergency department exhibiting mental change. The patient had a high fever for four and a whole-body rash for three days prior to his visit. He had displayed irritable symptoms and been vomiting for six hours before his visit, accompanied by seizure. After 13 hours of admission, the patient's right pupil became fixed and fully dilated, and the left pupil also became fixed and fully dilated within 30 minutes. Brain computed tomography (CT) was performed immediately, and severe brain swelling with transtentorial brain herniation was found. The mannitol dose was increased and dexamethasone was also added. Hyperventilation was performed through intubation to reach PaCO 2 levels of 25 to 30mmHg. Fifteen hours later, pupillary reflex was observed and the cerebral edema and transtentorial brain herniation was found to be improving in follow-up brain CT. He was transferred to a general ward after 11 days and discharged on the thirteenth hospital day without any neurological sequelae. Human herpesvirus type 6 (HHV-6) was detected in the serological polymerase chain reaction (PCR) examination.