Concerns remain regarding patient compliance with nasal continuous positive airway pressure (nCPAP). Poor objective compliance during the first months of treatment has been reported, but no data are available among chronically treated patients. Use of nCPAP, in 17 chronically treated obstructive sleep apnoea patients (820+/-262 days) was evaluated objectively using a pressure monitor (MC+; Sefam, France). Two consecutive recording periods of 30 sessions of treatment were scheduled at the patient's home. To minimize the potential bias caused by the introduction of the monitor, only the pressure data obtained at the end of the second period of recording (T2) were analysed. During the 28.1+/-2.6 monitored days, the mean effective daily rate of use was 7.1+/-1.1 h, 97% of the rate indicated by the standard in-built time counter. The prescribed pressure was observed during 95% of the machine run time. The nCPAP system was used for 94% of the monitored days. Sixty percent of the patients used their device every day. These preliminary results suggest that, contrary to reported compliance during the early period of the treatment, objective use of nasal continuous positive airway pressure therapy in chronically treated patients is satisfactory.