Using a randomized parallel group study design, we compared the cognit ive effects of carbamazepine (CBZ), phenobarbital (PB), and valproate (VPA) in children with epilepsy. Seventy-three children with newly diagnosed epilepsy were tested with the Wechsler Intelligence Scale for Children-Revised (WISC-R), Bender-Gestalt test, and auditory event-related potentials (P 300) before and 6 and 12 months after antiepileptic drug (AED) treatment. There were no significant differences in WISC-R IQs and Bender-Gestalt scores for children in any group at any of the three sessions. P 300 latencies were increased in the children receiving PB but not in children receiving CBZ and VPA. P 300 amplitudes were significantly reduced in treated children in all three groups, but amplitudes were not significantly different among the three groups. These findings suggest that PB may affect cognitive function of epileptic children and that the P 300 may be a sensitive additional procedure that can be used to assess the cognitive effect of AEDs.