A prospective study aimed at objectively evaluating compliance with nasal continuous positive airway pressure (CPAP) treatment was conducted in 728 obstructive sleep apnea [OSA; apnea/hypopnea index (AHI) > 15 events/hour] patients and 98 nonapneic snorers (AHI < or = 15 events/hour). Five-hundred seventy-five OSA patients and 33 nonapneic snorers underwent CPAP therapy and were followed-up for an average of 1,176 +/- 38 days (27 to 4,203 days). Compliance to treatment was measured by the mean rate of use of the CPAP device obtained from a built-in time counter. Acceptance of treatment was measured using Kaplan-Meier's model. The acceptance of CPAP was greater than 90% at 3 years and greater than 85% at 7 years in OSA patients. It was greater than 60% at 3 years in nonapneic snorers. The mean rate of CPAP use was 5.7 +/- 1.8 hours/day in OSA patients and 5.6 +/- 1.4 hours/day in snorers who were still on CPAP on October 1, 1995. It was correlated positively with age, body mass index, and AHI, and it was correlated negatively with daytime partial pressure of oxygen (PaO2), forced expiratory volume in 1 second (FEV1), and vital capacity in the group of OSA patients. This study shows that CPAP therapy is reasonably accepted by OSA patients as well as by nonapneic snorers. Both within and between groups, objective disease severity (as measured by the respiratory event index and daytime and nighttime hypoxemia), rather than patients' symptoms or complaints, seemed to play a role in the quality of compliance to treatment.