Interictal regional cerebral blood flow was determined in 11 adult patients with partial epilepsy and lateralized electroencephalographic abnormalities by means of the xenon 133 intracarotid injection method. A hemispheric area demonstrating significantly low regional cerebral blood flow values as compared with the hemispheric mean cerebral blood flow was observed in each of the patients. In 10 of the 11 patients the localized reduced regional cerebral blood flow levels deviated significantly from levels obtained from parallel regions and hemispheres of normal controls. In the majority of patients, the site of low regional cerebral blood flow closely correlated with the clinical type of partial seizures and/or the site of main electroencephalographic abnormality. On the basis of these data, we suggest that the epileptogenic focus responsible for the partial seizures may be localized within the hemispheric area demonstrating the abnormal regional cerebral blood flow reduction.