The long-term acceptability of treatment with nasal continuous positive airway pressure (CPAP) was studied prospectively in 44 patients with obstructive sleep apnea syndrome. At 14 months on the average after starting treatment with CPAP, 30 patients (68 percent) were found to be compliant (characterized by use of the apparatus every night throughout the night, for more than 5 h per night). The daily use of nasal CPAP was significantly correlated to the initial apnea/hypopnea index (p = 0.013; r = 0.37), as well as to the percentage of light sleep (p = 0.045; r = 0.30) and slow-wave sleep (p = 0.037; r = -0.31) during the initial polygraphic recording. We found a strong correlation between the daily use of nasal CPAP and the difference in the apnea/hypopnea index (p = 0.025; r = -0.34), the difference in mean oxygen saturation during sleep (p = 0.013; r = 0.38), and the difference in hypersomnia scores (p = 0.006; r = -0.40) obtained before and after treatment by nasal CPAP. Thus, patients used CPAP much more if they had an initial significant clinical handicap and if they were aware of the beneficial effects of CPAP. Under these conditions, patients tended to use the apparatus for the optimal length of time, regardless of the side effects linked to the treatment. This ensured efficacy and the maintenance of good compliance. This study confirms the importance of supervision of the time counter, as well as regular encouragement of patients to use the treatment as long as possible each night, in order to extract a maximum benefit from treatment by nasal CPAP.