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, education- matched normal controls (AHI=2.9+/-1.8/hr) underwent neuropsychological tests and ERP studies. RESULTS: Patients with severe OSAS showed deficits in corsi block forward and backward test during neuropsychological evaluation, and delayed latency and decreased amplitude of auditory P300. There were significant correlations between auditory P300 amplitudes and digit span forward or corsi block forward test scores, and between visual P300 amplitudes and digit symbol test scores. CONCLUSIONS: These findings suggest that severe OSAS patients may have deficits in attention and short-term memory, and abnormal auditory P300.