Surgical treatment of epilepsy is undertaken on the basis of prognostically favorable clinical and laboratory data. Analysis of neuropsychological test data obtained preoperatively and postoperatively from 14 surgically treated patients indicates that specific measures of neuropsychological test performance may provide prognostic information aside from the utility of this testing for lateralization and localization. An Impairment Index which is derived from the extensive test battery was related to change of seizure frequency. The groups of patients which have an improved or seizure-free status postoperatively also have the least preoperative neuropsychological impairment.The group of patients which did not show improvement of seizure frequency had a younger mean age at onset of seizures (&OV0335; = 6.7 years), while the group which was rendered seizure free had a later age at onset (&OV0335; = 17.2 years). There was no relationship for the duration of seizures prior to surgery and the outcome following surgery.