Landau-Kleffner syndrome (acquired epileptic aphasia) is characterized by language regression following normal acquisition of language skills, accompanied by epileptiform abnormalities on the electroencephalogram (EEG) with or without clinical seizures. Continuous spikes and waves during slow wave sleep may be seen on the EEG, but are not required to make the diagnosis. Structural neuroimaging with computed tomography (CT) and magnetic resonance imaging (MRI) is typically normal. We have evaluated 17 children (aged 2.4 to 10.6 yr) with Landau-Kleffner syndrome using positron emission tomography (PET) with 2-deoxy-2-[18F]fluoro-D-glucose (FDG) in order to determine whether there are metabolic abnormalities common to this syndrome. Patients were awake for the uptake period of FDG, and the EEG was monitored. On a visual analysis of the PET images, patients showed metabolic abnormalities in the temporal lobes. Two children had focal hypermetabolism in the left temporal cortex, one of whom also showed right temporal cortex hypometabolism. The remaining patients (n = 15) showed bilateral temporal hypometabolism, and comparison of these patients with a neurologically normal age-matched control group (n = 8) demonstrated significantly reduced glucose metabolism bilaterally in middle temporal gyrus (P < .02). In addition, other cortical regions displayed hypometabolism, although these regions were not consistently abnormal in all patients. The finding of temporal lobe abnormalities in all Landau-Kleffner syndrome patients suggests that temporal lobe structures are important in the pathophysiology of this syndrome, whereas the presence of additional cortical abnormalities in many patients indicates that extensive brain functional disturbances are common.