Acute encephalitis with thalamotegmental involvement in infants and children was reported in Japan, 1995. This encephalitis was preceded for several days by fever and symptoms of upper respiratory infection. These symptoms were followed by the rapid evolution of stupor and coma, associated with generalized seizures and decorticated and decerebrate rigidity without focal neurologic or meningeal signs. The prognosis was generally poor. We have experienced a case of acute encephalitis; the child had three days of fever before the hospitalization and convulsion and deteriorated mental change into a semicomatous state had developed. The patient was diagnosed as acute encephalitis with thalamotegmental involvement by magnetic resonance imaging.