The adverse effects of and adherence of a treatment with nasal continuous positive airway pressure (nCPAP) for obstructive sleep apnea and hypopnea syndrome (OSAHS) were examined. The subjects comprised 321 patients with OSAHS. An all-night polysomnography was performed for all subjects. Titration of CPAP was performed manually. Regarding the adverse effects of nCPAP therapy, the subjects most frequently complained of nasal symptoms, air-leaks and any trouble with the mask. Complaints heard less frequently were sleep disturbances, dry pharyngeal and oral cavities, noises, and pronounced pressure sensations due to CPAP. No adverse effects were noted in 13% of the subjects. The adherence was as high as 73.0% in 122 patients in whom time spent for nCPAP treatment could be analyzed. The mean treatment time was 4.7± 1.8 hours. We found no significant correlation between mean treatment time and age, BMI, ESS, CPAP, AHI, Ar-I, ODI, lowest SpO2 or %TST with SpO2< 90%. Multivariate analysis revealed no factor significantly correlated with treatment time. These results showed that detailed instructions for the prevention of adverse effects may improve the adherence of CPAP