Japanese B encephalitis is a disease with high mortality and many of those surviving suffer from serious sequelae. During the 1992 epidemic in northern Vietnam 50 patients treated at the Institute for Protection of Children's Health in Hanoi were studied concerning the type of sequelae and the development of the symptoms during the first two months of the disease. The age span was 1 to 15 years. 29 of the patients (58%) did not recover fully during the observation period. Fifteen (30%) showed signs of both neurological and mental disturbances. Nine (18%) only had mental symptoms while 5 (20%) suffered from isolated neurological sequelae. EEG was pathological in 9 out of 30 tested cases (30%); 9 of 23 patients (39%) performed subnormal IQ tests. Deep coma, bronchopneumonia with cyanosis, apnea attacks, prolonged fever and coma were all correlated (without statistical significance) to a higher risk for subsequent sequelae.