ObjectiveTo determine whether 1-week continuous positive airway pressure (CPAP) treatment, compared with placebo CPAP, improves cognitive functioning in patients with obstructive sleep apnea (OSA).Methods36 OSA patients (aged 32–60 years, respiratory disturbance index [RDI]>15) were monitored 2 nights with polysomnography, then randomized for 1-week treatment to CPAP or placebo (CPAP at 2 cm H2O with holes in mask). Participants completed Wechsler Adult Intelligence Scale—Revised Digit Symbol and Digit Span, Trailmaking A/B, Digit Vigilance, Stroop Color-Word, Digit Ordering, and Word Fluency tests pre- and posttreatment. These produced 22 scores per participant, which were analyzed by use of repeated-measures analysis of variance (ANOVA) and a rank-sum test.ResultsIn ANOVA, only 1 of the 22 scores showed significant changes specific to CPAP treatment, a number that could be expected by chance alone: Digit Vigilance-Time (p= .035). The CPAP group improved their time (from 7.5 to 6.9 minutes,p= .013). The rank-sum test revealed that the CPAP group had significantly better overall cognitive functioning posttreatment than the placebo group (mean ranks of 17.8 vs. 20.2, respectively;p= .022).ConclusionsAlthough results suggest overall cognitive improvement due to CPAP, no beneficial effects in any specific cognitive domain were found. Future studies of neuropsychological effects of CPAP treatment should include a placebo CPAP control group. Placebo studies that use longer-term treatment might demonstrate additional effects. It is also possible that, even at 2 cm H2O, CPAP conveys some beneficial neuropsychological effects.