A 50-year-old man with Sturge-Weber syndrome (SWS) and left cerebral hemisphere atrophy was studied using PET with O-15 oxygen and O-15 water in order to evaluate regional cerebral blood flow and energy metabolism in this condition. PET demonstrated decreased perfusion and increased oxygen extraction fraction in the affected left cerebral hemisphere and ipsilateral cerebellum. These findings suggest that a chronic state of reduced perfusion may be the etiology for the cerebral hemiatrophy seen in some patients with SWS.