Continuous positive airway pressure (CPAP) therapy improves daytime function in the sleep apnea/ hypopnea syndrome (SAHS) but it is unclear which patients benefit and what factors predict this improvement. To test the hypothesis that brief arousals from sleep predict improvements in daytime functioning with CPAP therapy, we prospectively studied 62 patients with polysomnography-defined SAHS. Each underwent daytime function assessments at baseline and after 6 mo of CPAP therapy to measure objective sleepiness, psychological well-being, quality of life, and cognitive performance. The microarousal frequency and AHI were poor predictors of improvements in daytime function with CPAP. Measures of hypoxemia predicted improvements in the mean sleep latency on the maintenance of wakefulness test, SAHS symptoms, quality of life, and reaction time, but such correlations were weak or moderate only explaining between 7% and 22% of variance. Significant relationships were found between CPAP use and improvements in self-ratings of daytime function. Results suggest that standard polysomnographic baseline variables are poor predictors of the response to CPAP therapy.