The aims of this study were to assess cognitive function in obstructive sleep apnea (OSA) patients and to evaluate the effect of short- and long-term treatment with continuous positive airway pressure treatment (CPAP). A battery of neuropsychological tests, the Epworth Sleepiness Scale (ESS), and the Beck Inventory Scale were administered to 23 patients with severe OSA (age: 56.5±6.13; AHI: 54.9±13.37) and to 23 age- and education-matched controls. The OSA patients were evaluated in a baseline condition and in two follow-up treatment sessions (after 15 days and 4 months of CPAP, respectively). At baseline, OSA patients had a significant impairment, compared to controls, in tests of sustained attention, visuospatial learning, executive function, motor performance, and constructional abilities. The longitudinal evaluation showed that after a 15-days CPAP treatment attentive, visuospatial learning, and motor performances returned to normal levels. A 4-months CPAP treatment did not result in any further improvement in cognitive tests. Performance on tests evaluating executive functions and constructional abilities was not affected by short- and long-term treatment with CPAP. The findings of this study confirm the hypothesis of partial reversibility of cognitive dysfunction in OSA patients after CPAP.