Continuous positive airway pressure (CPAP) is the treatment of first choice for obstructive sleep apnea syndrome (OSAS). The effect of this treatment is highly dependent on patient compliance. To identify factors associated with compliance, a questionnaire was sent to 53 patients with OSAS for whom CPAP treatment had been prescribed. Response rate was 96%. 45 of 51 patients (88%) stated they had been on CPAP for a period of 30 to 2340 (median 292) days. 43 of the 51 patients (84%) who used CPAP regularly, defined as at least 4 hours nightly on 5 nights per week, were considered as compliant with the prescribed treatment. They did not differ from non-compliant patients either in initial symptom score or in apnea/hypopnea index at initial presentation. The most consistent factor associated with compliance was symptomatic improvement with CPAP. Side effects were more common in non-compliant patients. Symptomatic improvement during an initial trial with CPAP should be considered when starting definitive treatment, in order to optimize compliance.