The true responsible lesion of Lennox-Gastaut syndrome was not adequately elucidated. Three cases of this syndrome were investigated by using Positron Emission Tomography (PET). PET revealed the differences in the left and right sides, especially the hypometabolism from the frontal to temporal regions. With this finding we supposed that the L-G syndrome implies one type of partial epilepsy caused by damage in parts of the temporal or frontal lobe.