Mouth breathing may contribute to increased collapsibility of the upper airways because of decreased contractile efficiency of the upper airway muscles as a result of mouth opening. Nasal airflow is also hypothesized to have a stimulating effect on the respiratory drive via receptors in the nose. Accordingly, patients with sleep-related breathing disorders (SRBD) are believed to breathe more through their mouth than healthy persons. In our department, polysomnography is routinely performed with a nasopharyngeal/oesophageal catheter for continuous pressure and flow measurements. We have shown previously that these sensors are able to differentiate between oral and nasal breathing. The aim of this study was to use these sensors to estimate the degree of oral breathing during sleep in patients with SRBD. The proportions of mouth breathing during sleep were 9.8% (SD 19.1%) for patients without SRBD, 16.4% (17.8%) for patients with mild SRBD, 11.9% (13.4%) for patients with moderate SRBD and 4.5% (7.4%) for patients with severe SRBD. There were no significant differences in the amount of mouth breathing between these groups. The proportion of patients who breathed through the mouth was smaller than that described previously for healthy subjects. The more severe the disease, the less patients tended to breathe through their mouth alone, and this finding was the opposite of what we expected.