Patients with sleep apnea/hypopnea syndrome commonly demonstrate impaired daytime performance. In a prospective study, 29 patients with sleep apnea/hypopnea syndrome were assessed polysomnographically to determine the relationship of cognitive performance and daytime sleepiness with sleep disruption, hypoxemia, and mood. Deterioration of cognitive performance correlated significantly with increasing severity of nocturnal breathing irregularity, magnitude of nocturnal hypoxemia, and extent of sleep disruption. Multiple regression analysis identified frequency of apneas plus hypopneas and of arousal and the extent of nocturnal hypoxemia as the variables most strongly associated with cognitive deficits. Anxiety and depression also contributed to this impairment. Objective daytime sleepiness was not significantly associated with nocturnal variables. This study showed that the frequency of breathing irregularities and the extent of both sleep disruption and nocturnal hypoxemia are important in determining daytime function in patients with sleep apnea/hypopnea syndrome. All of these factors should be considered when deciding which patients require treatment.