In a simulated monitoring situation, 21 anesthesia residents were tested for their ability to detect significant changes in four critical variables in the presence of a concurrent distraction. Each resident was tested after a night without clinical responsibility (rested) and after 24 hours of in-house call (fatigued). When fatigued, the residents scored significantly worse on the vigilance test than when rested (57.2 +/- 15.4 versus 65.9 +/- 10.9, P less than 0.02). Despite the small population size, the possibility of subject and investigator bias, and the artificial setting, these results support the intuitive proposition that a fatigued person is less likely than a rested person to detect important changes in monitored variables.