The study was conducted to evaluate the cognitive deteriorations induced by sleep deprivation with the computerized neurocognitive tests and the P300 event-related potential. Thirty healthy college students (22 men, eight women) participated in the present study. Subjects remained awake for 38 h under continuous surveillance. In the morning and the evening of the two study days, the computerized neurocognitive tests and the P300 were performed. In vigilance test and reaction unit test, there were significant cognitive impairments during sleep deprivation. However, in the cognitrone test there was significant functional improvement, which might be due to the practice effect. The P300 latency was significantly prolonged and the amplitudes decreased during sleep deprivation. The cognitive impairment during 38 h of sleep deprivation was mainly in terms of vigilance and reaction time. In contrast, higher complex cognitive function such as fine perceptual analyses, visual discrimination and working memory might be not affected by 38 h of total sleep deprivation. The changes of P300 were significantly correlated with the results of vigilance and reaction unit tests but not with the cognitrone test. Taken together, these results suggest that the P300 changes that occur during sleep deprivation are a reflection of the decrement in vigilance, which prolongs reaction time.