A patient had adversive seizures accompanied by unilateral pupillary dilatation. Neuropathologic examination showed a contusion involving the middle frontal gyrus contralateral to the pupil that dilated during the seizure. The clinical features and the location of this lesion (presumably the epileptogenic focus) correlate well with experimental studies in which pupillary changes have been observed during stimulation of the cerebral cortex. Unilateral mydriasis associated with contraversion of the eyes during seizures may have value in localizing epileptogenic foci.