A case is presented which represents the first instance of the use of functional positron emission tomography (PET) scanning to precisely localize a structural brain lesion to the precentral gyrus, and the first validation of functional PET scanning by intraoperative cortical mapping. The lesion was a 3-cm arteriovenous malformation (AVM) that had produced a generalized seizure in an otherwise asymptomatic young woman. A first, resting H2(15)O PET scan identified the AVM. A second PET scan, performed during vibrotactile stimulation of the contralateral hand, identified the somatosensory area of the hand region and localized the AVM to that part of the precentral gyrus immediately in front of it. This relationship and localization were confirmed by cortical mapping at the time of craniotomy under local anesthesia. Functional PET scanning may prove to be useful to localize cortical lesions precisely and to help in determining preoperatively the best form of treatment for lesions, especially AVM's, in functionally important cortex.