Obstructive sleep apnea syndrome (OSAS) is a sleep disorder characterized by repetitive episodes of upper airway obstruction. The aim of this study was to evaluate whether continuous positive airway pressure (CPAP) treatment is linked to improvements in cognitive abilities and emotional functions of patients with OSAS. Following the exclusion of four subjects for non-adherence to CPAP treatment, the final study sample was composed of 16 patients with moderate-to-severe OSAS, who were assessed both prior to and after 3 months of CPAP treatment, using a neuropsychological battery and questionnaires to assess mood and anxiety disorders, irritability, quality of life, quality of sleep and daytime sleepiness. We observed significant improvements in Digit Span Backward, Short Story and Corsi Span performances after 3 months of CPAP treatment. Questionnaires showed a significant reduction in daytime sleepiness and improvements in the subjective perception of sleep quality and sleep efficiency, and reduced daytime dysfunction. CPAP treatment has significant effects on different cognitive domains in patients with OSAS, especially working memory, long-term verbal memory, and short-term visuospatial memory.