Purpose: We investigated the difference of glucose metabolism of medial and lateral temporal lobes of patients with temporal lobe epilepsy (TLE) utilizing quantitative comparison of regional metabolic activities using asymmetric index. Materials and Methods: We studied 19 pathologically proven mesial TLE and 25 lateral TLE patients. Lateral TLE patients were either normal on magnetic resonance imaging (cryptogenic: n=14) or had structural lesions (lesional: n=11). Asymmetric index (ASI) was calculated as [(ipsilateral-contralateral)/(ipsilateral+ contralateral)]¡¿200. Results: ASI of medial and lateral lobes of mesial TLE was decreased (-16.4¡¾8.3 and -12.7¡¾5.5, respectively). In cryptogenic lateral TLE, ASI of lateral temporal lobe was decreased (-11.8¡¾4.7), whereas that of medial temporal lobe was not decreased (-4.6¡¾6.3). ASI of medial lobe of lesional lateral TLE was -7.3¡¾9.1, which was significantly different from that of mesial TLE (p<0.05). Patients with lesional lateral TLE had evident metabolic defects or decrease (ASI: -22¡¾10.5) in lateral temporal lobe. While we could not find the difference of metabolic activity in lateral temporal lobes between cryptogenic lateral TLE and mesial TLE patients, the difference of metabolic activity was significant in medial temporal lobes which was revealed by ASI quantitation. Conclusion: Asymmetric decrease of metabolic activity in both medial and lateral temporal lobes indicates medial temporal epilepsy. Symmetry of metabolic activity in medial temporal lobe combined with asymmetry of that in lateral temporal lobe may give hints that the epileptogenic zone is lateral.