The purpose of this preliminary study is to determine the profile of cognitive impairment of amyotrophic lateral sclerosis (ALS) using neuropsychological test batteries and event-related potential (ERP) measures. We examined seven sporadic ALS according to the El Escorial criteria; two patients had a spinal, five a bulbar onset of the disease. Neuropsychological signature was assessed by the Mini-Mental State Examination (MMSE) and the New Modified Wisconsin card sorting test (WCST). We also performed correlative studies with arterial blood gas analysis. The auditory “oddball” stimulation was presented and recorded from 20 channels, and the amplitude and peak latency of global field power (GFP) was determined. MMSE score, resulted in more than 24, revealed no obvious dementia of all patients. However, the preservative errors of WCST supposed to increase among bulbar-onset patients. There was no relation between neuropsychological scores and PaCO2/PaO2 level. Although the subgroup difference of ERP was not remarkable, P3 latency negatively correlated with PaO2 level and N1/P3 latency positively correlated with PaCO2 level. These findings suggest that a possible reduction of prefrontal function exists in the bulbar-onset patients, and hypoxia and hypercapnia may also influence to abnormalities of ERP components regardless of ALS subtype.