Cognitive functions may be altered in patients with obstructive sleep apnea syndrome (OSAS) and it has been proposed that vigilance and attention changes play a fundamental role in all aspects of cognitive deficits noted in this disease. The use of event-related potentials (ERPs) is a high-time resolution technique that can be used to explore the presence of cognitive dysfunction. We review 23 empirical articles on ERPs in OSAS in order to contribute to the clarification of the pattern of cognitive deficits that are specific to this disease and to see whether there might be an improvement of abnormal psychophysiological findings with continuous positive airway pressure (CPAP) treatment. We conclude that ERP studies have contributed to demonstrating changes in cognitive attentive processing in OSAS, mainly in association with altered functioning of the prefrontal cortex, and that CPAP treatment may improve vigilance and attention and generally improve cerebral information processing in these patients. The remaining deficits during sufficient CPAP therapy may, however, reflect irreversible hypoxic cerebral damage.