The authors compared cognitive changes in epilepsy patients with or without antiepileptic drugs using event-related potential (ERP) N270 and P300. The patients were divided into three groups according to treatment type: carbamazepine, valproate acid (VPA), or no treatment. One control group composed of healthy subjects was included. The Mini-Mental State Examination detected no clinical dementia in all subjects. A modified Sternberg paradigm was used as a task when an ERP was recorded. When a stimulus was different from the memorized item (conflict condition), N270 was elicited in both controls and patients. All the three patient groups showed a delayed and smaller N270 than the control group. P300 was elicited when a stimulus was the same one as the memorized item (match condition), and its amplitude decreased in the VPA group. P300 latency did not differ among the four groups. ERP (N270 and P300) was abnormal in patients with epilepsy. These results indicate that antiepileptic drugs, especially VPA, might enhance the impairment of cognitive processing. N270 is more sensitive than P300 in the early detection of minor cognitive impairment in epileptics.