Classic phenylketonuria (PKU) is characterized by severe mental retardation in untreated individuals and mild neurocognitive abnormalities in some early treated adults. The exact biochemical mechanisms underlying this neurotoxicity remain undetermined. Several theories implicate abnormal cerebral energy utilization and alterations in biochemical pathways that involve glucose metabolism. This pilot study was undertaken to investigate whether 18F-deoxyglucose positron emission tomography (PET) is an effective tool to study cerebral glucose metabolism in early treated PKU. After PET coregistration with SPGR MRI, relative glucose metabolic rates (rGMR) at the center of standard atlas positions was determined. Repeated measures MANOVA was used to assess regional metabolic differences, which were then correlated with age-specific and day-of-scan plasma phenylalanine and age. Patients with PKU in comparison to controls had decreased rGMR in cortical regions including the prefrontal, somatosensory, and visual cortices, and increased activity in subcortical regions including the striatum and limbic system. Day-of-scan phenylalanine correlated with abnormal activity in subcortical structures, and older age was associated with decreased activity in the prefrontal and visual cortices. The clinical significance of these abnormalities of glucose metabolism in specific areas of the brain remains unknown.