The cases of five children with malign changes in seizure patterns during treatment with antiepileptic drugs are reported. Three patients developed irreversible Lennox syndrome. All the patients were treated by the association phenobarbital-diphenylhydantoin. The mechanism of the clinical and EEG modiifcations might be related to the development of an epileptic disturbance which has an opposite behaviour to those commonly improved by some antiepileptic drugs. The incidence of previous cerebral lesions is an important factors on the establishment of severe epileptic complications.