We employed positron emission tomography (PET) with 2-deoxy-2[18F]fluoro-D-glucose (FDG) to study local cerebral glucose utilization in 15 children who had Lennox-Gastaut syndrome. Our results show that LGS can be classified into four predominant subtypes, each with a distinct metabolic pattern: unilateral focal hypometabolism, unilateral diffuse hypometabolism, bilateral diffuse hypometabolism, and normal. Functional disturbances seen on FDG-PET did not always correlate with abnormalities revealed by x-ray computed tomographic scan. This classification of Lennox-Gastaut syndrome into four major metabolic subtypes not only provides a new perspective toward understanding cerebral function in this complex syndrome, but may also prove useful in the clinical management of these patients.