Features of seizures in children with acute lymphoblastic leukaemia were examined in relation to the type of treatment received for central nervous system prophylaxis. Of the 1289 patients in the study, 132(10%) had experienced one or more seizures. In 96 the seizures had not been associated with any recognisable aetiology and, with 3 exceptions, had been generalised or focal motor in type. Status epilepticus had occurred in one-third of the group with seizures. Initial seizure rates among patients in continuous complete remission were significantly related to the method of central nervous system prophylaxis. Those who had received intrathecal methotrexate repeatedly with or without moderate doses of methotrexate intravenously or prior prophylactic cranial irradiation had a 20-fold higher seizure risk during remission induction, and a 37-fold higher risk during the 6th to 12th month of therapy, than did those who had received irradiation and only five intrathecal injections of methotrexate early in the course of treatment. Other clinical factors associated with an increased frequency of seizures were escalation of intravenous methotrexate dosage following cranial irradiation and treatment of meningeal leukaemia with intrathecal methotrexate. Parenterally administered methotrexate thus seems to increase susceptibility to seizure development in childhood leukaemia patients.