Obstructive sleep apnea syndrome (OSAS) causes numerous pathophysiological changes and influences neuropsychological system. Nasal continuous positive airway pressure (CPAP) is the gold standard treatment of OSAS. We aimed to evaluate the compliance of patients with severe OSAS to CPAP treatment and to compare the anxiety and depression scores before and at the 6th month of the treatment. Seventeen patients recieving CPAP treatment for OSAS that continued regular follow-up were accepted to the study. A questionnaire containing questions about demographic features, OSAS symptoms and Epworth sleepiness scale, Beck anxiety scale and depression scale was filled by the patients before and after CPAP treatment. Additionally, information concerning adverse effects of and compliance to CPAP treatment was optained after CPAP treatment. Mean age of the patients was 48.4 +/- 8.4 (32-63); 14 (82.4%) of them were male and 3 (17.6%) of them were female. Snoring, witnessed apnea and excessive daytime sleepiness symptoms, and Epworth sleepiness scale scores were significantly decreased after CPAP treatment compared with those before treatment. Mean duration of CPAP device use at night was 5.6 +/- 2.1 (2-8) hours. Nine (52.9%) of the patients were using the device regularly every night and 8 (47.1%) of the patients were using it irregularly. The most frequent adverse effect of CPAP treatment was sore mouth, followed by mask discomfort and erythema on the nose. Anxiety and depression scores of the patients were significantly decreased after CPAP treatment compared with those before the treatment. As a conclusion, OSAS patients with high anxiety and depression scores benefit from CPAP treatment.