Reye syndrome is a rapidly progressive encephalopathy with hepatic dysfunction, which often begins several days after apparent recovery from a viral illness, especially varicella or influenza A or B. Salicylate use was identified as a major precipitating factor for the development of Reye syndrome. With the recommendation to avoid use of salicylates in children, Reye syndrome has virtually disappeared in recent years. We report a case of Reye syndrome in a 5-month-old infant who had been treated with intravenous immunoglobulin and aspirin under the diagnosis of Kawasaki disease, and showed symptoms of sudden onset of irritability, rigidity, decreased activity, vomiting, poor appetite, lethargy, liver dysfunction without jaundice, coagulopathy, and hyperammonemia.