In this study, we evaluated the sensitivity of a 5-min personal digital assistant—psychomotor vigilance test (PDA—PVT) to severe sleep loss. Twenty-one participants completed a 10-min PVT-l92 and a 5-min PDA—PVT at two hourly intervals during 62 h of sustained wakefulness. For both tasks, response speed and number of lapses (RTs > 500) per minute significantly increased with increasing hours of wakefulness. Overall, standardized response speed scores on the 5-min PDA—PVT closely tracked those of the PVT-192; however, the PDAPVT was generally apsociated with more lapses/minute. Closer inspection of the data indicated that when the level of sleep loss and fatigue became more severe (i.e., Day 3), the 5-min PDA—PVT was not quite as sensitive as the 10-min PVT-l92 when 2-to 10-sec foreperiods were used for both. It is likely, however, that the observed differences between the two devices was due to differences in task length. Thus, the findings provide further evidence of the validity of the 5-min PDA—PVT as a substitute for the 10-min PVT 192, particularly in circumstances in which a shorter test is required and/or the PVT-l92 is not as practical.