Background: Patients with obstructive sleep apnea syndrome (OSAS) show variety of dysfunctions in cognitions including general cognitive function, attention, and frontal lobe and executive function. However, there is no consensus on the main features of the cognitive dysfunction in OSAS patients. So we performed neuropsychological tests and event-related potential (ERP) studies in patients with severe OSAS to evaluate the cognitive dysfunctions and changes of auditory and visual P300. Methods: Twenty-eight men with severe OSAS (apnea hypopnea index (AHI)=63.1±}17.8/hr) and 16 age, sex, educationmatched normal controls (AHI=2.9±}1.8/hr) underwent neuropsychological tests and ERP studies. Results: Patients with severe OSAS showed deficits i...