Poor localization of the interictal epileptic abnormality and the rather unreliable EEG localization of ictal onsets in patients with frontal lobe epilepsy may be due to some of the following factors: (1) high risk of EEG sampling error, (2) early seizure spread within and outside the frontal lobe, (3) widespread distribution of the epileptogenic brain tissue responsible for the patient's habitual seizures, and (4) secondary bilateral synchrony and secondary epileptogenesis. The relevance of these factors in the preoperative investigation of patients with frontal lobe epilepsy was studied in: (a) 34 adult patients (mean age: 25 years) who became and persisted seizure free after restrictive surgical removal of the anterofrontal region, the parasagittal region convexity or the fronto-opercular area; (b) 12 patients in whom the pre-operative EEG investigation with extracranial electrodes failed to localize the site of seizure onset, leading to further investigation with depth electrodes.