We used positron emission tomography with fluorine 18-labeled 2-deoxyglucose to study cerebral glucose metabolism in 10 patients with Lennox-Gastaut syndrome who had normal neuroradiological studies. The scans showed decreased metabolic rates relative both to those in the caudate nucleus and to normal control values in 3 patients whose seizures began before the age of 1, as well as in a patient with hyperprolinemia. No patient had a region of persistent focal hypometabolism. Metabolic rates increased in parallel with increased electroencephalographic discharges in 1 patient; 3 patients had lower metabolic rates when the electroencephalogram showed epileptiform discharges and while the patients were taking barbiturates.