Positron emission tomography (PET) of local cerebral glucose metabolism in 13 children with infantile spasms of undetermined cause (cryptogenic spasms) revealed unilateral hypometabolism involving the parieto-occipito-temporal region in 5 female infants. Cranial computed tomography showed normal findings in all infants. Magnetic resonance imaging (MRI) demonstrated a normal appearance in 4 of the 5 infants; in 1 infant, MRI revealed a subtle abnormality consisting of poor demarcation between occipital gray and white matter. Surface electroencephalography (EEG) in 4 showed hypsarrythmia at some time in the patients' courses, but at other times showed localized or lateralized abnormalities corresponding to areas of PET-detected hypometabolism. Because of poor seizure control, 4 infants underwent surgical removal of the cortical focus guided by intraoperative electrocorticography and were seizure free postoperatively. Neuropathological examination of resected tissue in each showed microscopic cortical dysplasia. Our findings indicate that in infants with cryptogenic spasms, PET can effectively identify those due to unsuspected focal cortical dysplasia, for which resective surgery offers improved prognosis.