In sleep apnea syndrome (SAS), vigilance impairment is typically associated with highly disrupted sleep, but recently, nocturnal hypoxemia has also been identified as a second pathogenetic factor in patients with a high degree of desaturation. However, although sleep disruption has been demonstrated to play a role in both the propensity to fall asleep and the capacity to stay awake, the role of nocturnal hypoxemia has been implicated only in the latter. In the present study, both sleep disruption and nocturnal hypoxemia were assessed in 20 moderately to severely apneic patients. During the day, vigilance was assessed both by the multiple sleep latency test (MSLT), as a measure of the propensity to fall asleep, and by the four-choice reaction time test (FCRTT), as a measure of the capacity to stay awake in a performance task. Severity of nocturnal hypoxemia was found to predict performance on the MSLT, as well as on the FCRTT, but sleep disruption was found to predict performance only on the FCRTT. These results suggest that in moderately to severely affected SAS patients, nocturnal hypoxemia may play a primary role in the pathogenesis of vigilance impairment. (Chest 1991; 100:367-70)