BACKGROUND Patients with obstructive sleep apnea syndrome (OSAS) often complain of dryness of mouth and throat prior to and during nasal continuous positive airway pressure (nCPAP). It is believed that this is due to mouth breathing (MB). However, the association between mouth breathing and apneas/hypopneas and the effect of CPAP on MB has not been studied. OBJECTIVE The purpose of the present study was, therefore, to assess the frequency and duration of episodes of MB prior to and during treatment with nCPAP. METHODS MB was recorded prior to and during nCPAP with a closely fitting mouth mask connected to a pneumotachograph and nasal flow was measured via nasal prongs. MB episodes were expressed as the number of events divided by total sleep time x 60, to give the MB event index per hour of sleep. MB time divided by total sleep time x 60 was calculated in minutes to get the MB time index per hour of sleep. METHODS Eleven male patients with OSAS (mean age 57.9 +/- 8.3 years, body mass index 30.2 +/- 3.8) were recruited to the study. RESULTS Prior to nCPAP, the apnea/hypopnea index was 55.8 +/- 26 and decreased during nCPAP to 8.0 +/- 3.4. The lowest SaO2 measured was 82.9 +/- 4.7%, and increased to 87.5 +/- 2.7% under nCPAP. The mean nCPAP was 7.8 +/- 1.6 cm H2O. MB event index per hour of sleep decreased from 35.2 +/- 19.7 prior to treatment to 5.0 +/- 5.2 under nCPAP (p < 0.01). In 52.2 +/- 27.4% of obstructive respiratory events, MB started at the end of an apnea/hypopnea episode, decreasing to 8.5 +/- 12.5% with nCPAP treatment. MB time index per hour of sleep was reduced from 13.5 +/- 10.2 min prior to treatment to 4.6 +/- 5.5 min under nCPAP (p < 0.05). CONCLUSIONS In OSAS patients, MB episodes often appear at the termination of an apnea/hypopnea episode. In many cases, MB episodes can be markedly reduced by nCPAP treatment. When patients on nCPAP complain of dry mouth, appropriate measurements should be performed to verify MB.