Influenza encephalopathy and related disorders such as Reye's syndrome, acute necrotizing encephalopathy and acute hemorrhagic encephalopathy are still undefined clinical entities because the pathophysiology is not clearly established. Even Reye's syndrome which has been extensively studied during the last two decades, its real picture has not been well established. Moreover, Reye's syndrome is going to disappear just like Ekiri syndrome. Acute necrotizing encephalopathy and acute hemorrhagic encephalopathy need further investigation to be recognized as single clinico-pathological entity because decreased cerebral blood flow is closely linked with brain edema including basal ganglia lesion and hepato-renal dysfunction.