Positron emission tomography with simultaneous electroencephalographic monitoring was performed with [18F]fluorodeoxyglucose in 20 patients with complex partial seizures who had normal computed tomographic scans. Seven patients had only unilateral epileptiform discharges on the electroencephalogram, 3 had predominantly unilateral discharges, and 10 had nonlocalized epileptiform abnormalities. Positron emission tomography showed a hypometabolic lesion in 16 of the 20 patients. Pathological changes in the hypometabolic region were found in postoperative specimens in 4 of 5 patients studied. Positron emission tomography was unaffected by the seizure frequency, state of alertness, or number of spike discharges during the scan. There was a tendency for patients to have higher overall metabolic rates when taking less medication. Seizures occurring during [18F]fluorodeoxyglucose uptake in 3 patients produced a hypermetabolic area at the interictal hypometabolic focus. Positron emission tomography sometimes showed more widespread hypometabolism than suspected on the basis of the scalp-recorded electroencephalogram. The frontal lobe showed a greater degree of hypometabolism than the temporal lobe in 3 patients. Focal lesions may be identified by positron emission tomography even if the electroencephalographic abnormality is not well localized.