PURPOSE: This study was designed to assess the clinical manifestations and the effect on prognosis of seizures in children with acute lymphoblastic leukemia(ALL). METHODS: The study group consisted of 20 patients(10 males and 10 females) who experienced seizures out of the 198 pediatric ALL patients(117 males and 81 females) who were diagnosed and treated at the Department of Pediatrics, St. Mary's Hospital, the Catholic University of Korea. RESULTS: The overall incidence of seizure developing after the diagnosis of ALL was 10.1% and in 11 patients(5.6%), seizure recurred one or more times. An average of 6.5+/-3.6 months(range 0-42 months) elapsed between the beginning of treatment and the new onset of seizures. In 13 patients(65%), the first seizure occurred during the induction or re-induction chemotherapy phase. Seizure type was partial seizure in 11 patients(55%), and generalized seizure in 9 patients(45%). In 15 patients(88.2%), brain imaging study showed abnormal findings and in 18 patients(90%), EEG revealed abnormal findings. 11 patients were diagnosed with epilepsy and were treated with long-term anticonvulsants. In these patients, 4 patients(36.4%) had no seizure recurrence, but 2 patients(18.2%) showed no response to anticonvulsants. The 5-year survival rate of the patients experiencing seizures was 47%, while the rate was 78% for those did not experience seizures (P<0.001). CONCLUSION: Seizures in pediatric patients with ALL was closely related to the 5-year survival rate. Therefore, these patients require early careful observation, evaluation and intensive care. Also, further studies such as ways to diminish the side effects of antileukemic agents are necessary to reduce the risk of seizure.