Aim: The aim of this study was to identify the clinical features of patients with obstructive sleep apnea syndrome ( OSAS) and investigate the impact of continuous positive airway pressure ( CPAP) treatment on nocturnal urine volume. Materials and Methods: This study enrolled 53 patients with moderate-to-severe OSAS and an apnea-hypoxia index of >20/hr. Data were collected on serum brain natriuretic peptide ( BNP) level, International Prostate Symptom Score-Quality of Life ( IPSS-QOL) score, Overactive Bladder Symptom Score ( OABSS), International Consultation on Incontinence Modular Questionnaire-Nocturia QOL ( ICIQ-NQOL) score, Epworth Sleepiness Scale ( ESS) score, and the frequency volume chart. Only patients who continued CPAP treatment for 3 months were included in the analysis. Results: In total, 40 patients ( 33 men and 7 women) completed the study ( 75.5%). The mean age was 56.9 years. The night-time frequency to void was significantly decreased from 2.1 to 1.2 after CPAP treatment ( P<0.01). The mean scores as assessed by ESS, IPSS-QOL, OABSS, and ICIQ-NQOL significantly improved after CPAP ( P<0.01). The mean diastolic blood pressure significantly decreased after CPAP treatment. However, there was no significant change in the BNP level before and after CPAP treatment. In the frequency volume chart, hours of undisturbed sleep, total nocturnal voided volume, and nocturnal polyuria index significantly improved after CPAP treatment ( P<0.05). However, 24-hr voided volume and mean voided volume during night-time did not change after CPAP treatment. Conclusion: CPAP treatment decreases night-time urinary frequency by reducing nocturnal urine production and improves QOL in patients with OSAS. (C) 2015 Wiley Periodicals, Inc.