'Double cortex' is a neuroblast migrational disorder characterized by a diffuse band of heterotopic grey matter between the lateral ventricles and cerebral cortex which may be normal or macrogyric. The authors report two girls with 'double cortex' syndrome presenting intractable partial epilepsy and severe mental retardation. EEG data are analysed in detail because such patients presented a particular interictal EEG background activity, not only with very stable features during the different stages of vigilance, but also uninfluenced by seizure frequency or duration. The authors raise the possibility that a further in vivo diagnostic parameter for this syndrome has been identified.