We report an investigation of P300 measures of information processing in patients with generalized epilepsy of the absence type and those with complex partial epilepsy. Studies have demonstrated that absence patients perform more poorly than complex partial patients on behavioral tests of sustained attention (the Continuous Performance Test, or CPT). Duncan [Duncan, C.C., 1988. Application of event-related brain potentials to the analysis of interictal attention in absence epilepsy. In: Myslobodsky, M.S., Mirsky, A.F. (Eds.), Elements of Petit Mal Epilepsy. Peter Lang, New York, pp. 341-364] reported that P300 was significantly reduced in a group of absence patients as compared with healthy controls. The present investigation was undertaken to compare the attention deficit in absence patients to that in complex partial seizure patients. Thus, ERPs were recorded while participants with absence seizure disorder, complex partial seizure disorder, and healthy controls performed auditory and visual versions of the CPT. A significant reduction in the amplitude of P300 on the visual CPT was observed in both groups of seizure patients as compared to controls. In contrast, P300 on the auditory CPT was reduced only in the group with absence seizures. These ERP data support and amplify previous behavioral findings of the impaired capacity of absence patients to mobilize and sustain attentional resources. Auditory sustained attention seems to be more affected by the pathophysiology of absence epilepsy than visual attention. Two possible factors may be involved: (a) There are separate visual and auditory attention systems in the brain, and the latter is more vulnerable than the former [Duncan, C.C., Kosmidis, M.H., Mirsky, A.F., 2005. Closed head injury-related information processing deficits: An event-related potential analysis. Int. J. Psychophysiol. 58, 133-157]; and (b) Auditory processing depends on intact mechanisms in the brainstem, which are dysfunctional in patients with absence seizures.