Introduction: Mortality rates in pediatric encephalopathy with fulminant cerebral edema and transtentorial brain herniation were reported as high as about 30%. We report a case of a 4-year-old boy who had encephalopathy with severe cerebral edema but recovered without any neurological sequelae. Case: A 4-year-old boy visited our emergency department after 3 minutes of seizure. He presented fever for the last 4 days and irritability with vomiting for 6 hours before he arrived. He appeared drowsy and vital signs were as follows: blood pressure of 105/70mmHg, heart rate of 112/min, respiratory rate of 24/min, and 37℃ of body temperature. In laboratory studies, WBC was 4,690/ mm3 (seg 83%) and CRP was 1.19mg/dL. Brain MRI was performed, and it showed diffusion image with high signal intensity (SI), ADC image with iso SI in both cerebral hemisphere cortex and diffuse cortical swelling. He admitted to the pediatric intensive care unit and empirical therapies with acyclovir, ceftriaxone, vancomycin and mannitol were initiated. After admission, there was no seizure but he progressed to stupor. After 13 hours of admission, his right pupil became fixed and full dilated, and then left pupil also became fixed and full dilated in 30 minutes. Brain CT was performed immediately, severe brain swelling with transtentorial brain herniation was found. Dose of mannitol was raised and dexamethasone was also added. Endotracheal intubation was performed for hyperventilation treatment to reach PaCO2 levels of 25 to 30mmHg. Fifteen hours later, normal pupillary reflex was observed and improving state of cerebral edema and transtentorial brain herniation was found in the follow-up brain CT. We stopped hyperventilation therapy on the 6th hospital day. We found diffusion image with multifocal high SI and ADC image with iso SI in right posterior parahippocampal gyrus, left thalamus, cortex of both uncus, and orbital gyrus in addition to markedly decreased brain edema in the follow-up brain MRI on the 7th hospital day. He was transferred to general ward on the 11th hospital day and discharged on the 13th hospital day without any neurological sequelae. Human herpesvirus 6 (HHV-6) was detected in the serological PCR examination. Conclusion: We report a rare case of HHV-6 encephalopathy with severe cerebral edema in a 4-year-old patient who recovered without any neurological sequelae.