We performed positron emission tomographic (PET) studies with 2-deoxy-2[18F]fluoro-D-glucose in 8 children with hemimegalencephaly (HME). HME is a developmental brain malformation associated with epilepsy, hemianopsia, and varying degrees of developmental delay. We hypothesized that the relatively poor overall developmental outcome of surgically hemispherectomized HME patients as a group, compared to children undergoing hemispherectomy for Sturge-Weber syndrome or chronic focal encephalitis, is related to dysfunction of the structurally "normal" non-HME side and that PET would be helpful in the pre-surgical evaluation of HME patients with intractable epilepsy. Visual analysis of the non-HME side on PET clearly revealed evidence of cortical hypometabolism in 4 patients compared to controls. Seven children underwent epilepsy surgery. One child had a glucose metabolic pattern suggesting a cortical lamination defect in the non-HME hemisphere, bilateral independent seizure onset, and was not considered to be a surgical candidate. We found a general correlation between the pattern of glucose utilization in the less affected hemisphere and prognosis. Although the follow-up periods are short, it is recommended that HME children with intractable epilepsy undergo hemispherectomy in the first year of life in order to allow maximal brain plasticity to occur; however, preoperative evaluation should also include an assessment of the integrity of the non-HME hemisphere.