We experienced an unusual case of acute encephalopathy in a 4 month-old boy He was admitted to our hospital because of lethargy and seizures after preceding symptoms of upper respiratory tract infection a few days before admission. On admission, he was in semicomatous mental state with decorticated rigidity. Laboratory tests showed normal blood sugar, ammonia, and transaminase levels. CSF was acellular and sterile. Brain MRI in both 71 and 72 weighted-image showed high signal density in both thalami and caudate nucleus head. After recovery, neurologic sequales of developmental delay, mental retardation, right hemiplegia and seizure remained. We report a case of acute encephalopathy that have clinical course similar to Reye syndrome but have specific brain image.