Purpose: As mesial temporal lobe epilepsy (TLE) shows hypometabolism of medial and lateral tempora lobe, we investigated whether symmetric uptake of F-18-FDG in medial temporal lobes can differentiat mesial from lateral TLE. Materials and Methods: In 113 patients (83 mesial TLE, 30 lateral TLE) wh underwent anterior temporal lobectomy and/or corticectomy with good surgical outcome, we performe F-18-FDG PET and compared F-18-FDG uptake of medial and lateral temporal lobes. All the patients wi mesial TLE had hippocampal sclerosis except one congenital abnormal hippocampus. Patients with latera TLE revealed cerebromalacia, microdysgenesis, arteriovenous malformation, old contusion, and cortical dys plasia. Results: Sensitivity of F-18-FDG PET and MR for mesial TLE was 84% (70/83) and 73% (61/83 respectively. Sensitivity of F-18-FDG PET and MR for lateral TLE was 90% (27/30) and 66% (20/30 respectively. Twelve patients were normal on F-18-FDG PET. 101 patients had hypometabolism of latera temporal lobe. Among 88 patients who showed hypometabolism of medial temporal lobe as well as later temporal lobe, 70 were mesial TLE patients and 18 were lateral TLE on pathologic examination. Positive predictive value of medial temporal hypometabolism for mesial TLE was 80%. Among 13 patients wh showed hypometabolism of only lateral temporal lobe, 4 were mesial TLE and 9 were lateral TLE. Positive predictive value of hypometabolism of lateral temporal lobe for the diagnosis of lateral TLE was 69% ( 13). Normal MR findings stood against medial TLE, whose negative predictive value was 66%. Conclusion Lateral temporal lobe epilepsy should be suspected when there is decreased F-18-FDG uptake in latera temporal lobe with normal uptake in medial temporal lobe. (Korean J Nucl Med 1999;33:111-9)