Seventeen patients with partial epilepsy had electroencephalographic (EEG) monitoring concurrent with cerebral positron emission computed tomography (PECT) after 18F-fluorodeoxyglucose (18FDG) and 13N-ammonia (13NH3) were given intravenously as indicators of local cerebral glucose utilization (LCMRglc) and relative perfusion, respectively. In 12 of 15 patients who had unilateral or focal electrical abnormalities, interictal 18FDG scan patterns clearly showed localized regions of decreased (14 to 58%) LCMRglc that correlated anatomically with the eventual EEG localization. These hypometabolic zones appeared normal on x-ray computed tomography in all but 3 patients and were unchanged on 18FDG scans repeated on different days. In 5 of 6 patients who underwent anterior temporal lobectomy the interictal 18FDG scan correctly detected the pathologically confirmed lesion as a hypometabolic zone, and removal of the lesion site resulted in marked clinical improvement. In contrast, the ictal 18FDG scan patterns clearly showed foci of increased (82 to 130%) LCMRglc that correlated temporally and anatomically with ictal EEG spike foci and were within the zones of interictal hypometabolism (three studies in 2 patients). 13NH3 distributions paralleled 18FDG increases and decreases in abnormal zones, but 13NH3 differences were of lesser magnitude. When the relationship of 13NH3 uptake to local blood flow found in dog brain was applied as a correction to the patients' 13NH3 scan data, local alterations in perfusion and glucose utilization were usually matched in both the interictal and the ictal state. We conclude that the interictal 18FDG-PECT scan is useful in aiding localization of the dysfunctional cerebral zone most likely to be responsible for seizures in patients being considered for anterior temporal lobectomy. With further development, emission computed tomography may help in better categorizing the various forms of the disorder and in elucidating the basic mechanisms of epilepsy in humans.