The case records for 256 patients with acute encephalitis, including meningoencephalitis, acute encephalopathy and Reye syndrome, were obtained from 36 departments of pediatrics in large hospitals through questionnaires which were sent in 1990-92. The incidence rate per 100,000 child years was 3.3, and was more frequent in children aged 0-4 years (rate 6.6) than 5-15 years (rate 2.0), with statistical significance. The disease in the 105 etiologically diagnosed patients was due to measles virus (24), herpes simplex (21), rubella (24), and other agents (36), including two cases of human herpesvirus 6. A third of the infants with measles encephalitis were first seen at less than 18 months of age. Eighteen cases of rubella encephalitis occurred during an epidemic of rubella in Aichi Prefecture, 1987-88. The short-term outcome (mean length of follow-up: 2 years 3 months) of encephalitis was death in 20 cases (7.8%) and sequelae in 58 (24%). Eleven patients died within 7 days and five by the 8-14th day from onset. Thirty-six children had multiple residual impairments, including twelve who were severely handicapped. The prognosis for both life and sequelae was significantly poor for herpes simplex encephalitis in etiologically diagnosed patients. We stress the importance of an increase in the vaccination rate and the establishment of an early diagnostic system.