We reviewed the records of 127 consecutive pediatric patients with acute lymphoblastic leukemia (ALL) to determine the incidence, timing, etiologies, and recurrence rate of seizures in this population. Patients with ALL and seizures were identified retrospectively by review of the records of all pediatric ALL patients who were diagnosed and treated during the years 1983 through March 1993 in a large tertiary-care hospital. Seventeen patients (13%) developed one or more seizures. In 16 patients, seizures occurred during antileukemic treatment, and in almost all of them seizures were related to intrathecal methotrexate (IT MTX) or subcutaneous L-asparaginase treatment. One patient who developed a seizure while not receiving chemotherapy had a history of cerebral infarctions. In 8 patients, (47%), the initial seizure episode was associated with a cerebral lesion. One or more seizures recurred in 6 patients. Four of these patients had an isolated recurrence, in 3 patients < or = 3 months and in 1 patient < or = 6 months after the initial event. Two patients (12%) with static encephalopathy and neurological deficits developed a chronic seizure disorder. There is a significant risk of acute symptomatic seizures in pediatric ALL patients. Most seizures in these patients occur during the acute treatment phase and are most frequently related to side effects of chemotherapy. The long-term recurrence risk is low; recurrence occurs most often in patients with evidence of cerebral structural lesions and neurological deficits. Long-term antiepileptic drug (AED) therapy should be restricted to such patients.