We evaluated the clinical role of positron emission tomography (PET) in 23 children with tuberous sclerosis complex. Mean age of the children when first scanned was 3.3 years. Mean age when seizures began was 8.7 months. All, except three, were at least mildly developmentally delayed. PET images were visually analyzed and compared to computed tomography (CT), magnetic resonance imaging (MRI), and the electroencephalogram (EEG). In two infants, interictal PET study was normal. One of the studies was performed with a low resolution early generation scanner at age 7 months; the other infant was 2 days old. Twenty-one of the 23 children had focal or multifocal cortical hypometabolism. Some hypometabolic cortical regions on PET did not show corresponding abnormalities on CT and MRI, and may be due to epileptogenic mechanisms or small tubers. PET provides additional localizing information to CT and MRI in patients with tuberous sclerosis complex. However, because of the normally low cerebral glucose metabolism in infancy, PET may give false negative findings if performed prior to about 1 year of age. The usefulness of glucose metabolism PET in most patients with tuberous sclerosis complex is limited. However, if the EEG, CT, and MRI abnormalities are unifocal or unilateral, and surgery is being contemplated, more detailed evaluation with PET may help to determine if contralateral tubers are present and evaluate the functional integrity of the brain as a whole.