In 3 children who had undergone cerebral hemispherectomy (hemidecortication) between the ages of 1 year 5 months and 4 years for the alleviation of intractable epilepsy, cerebral glucose utilization was studied serially with positron emission tomography. Three to 7 months after hemispherectomy, glucose utilization in the caudate nuclei on the side of hemispherectomy had decreased to below preoperative values, presumably due to total deprivation of ipsilateral cortical input. One to 2.5 years after surgery, complete restoration of glucose metabolic activity to preoperative levels was seen in 2 patients and partial recovery was seen in 1 patient. These alterations of cerebral glucose utilization are believed to reflect microscopic anatomical reorganizational changes (e.g., collateral sprouting) that have been documented following similar lesions in several animal models. Our findings suggest that positron emission tomography may provide a sensitive measure of developmental brain plasticity in vivo.