BACKGROUND Obstructive sleep apnoea is a common condition. Treatment with nasal continuous positive airway pressure (CPAP), while effective and safe, causes nasal congestion and stuffiness in some patients. The hypothesis that this study aimed to test was that nasal CPAP with a mouth leak and subsequent unidirectional airflow across the nasal mucosa causes an increase in nasal mucosal blood flux and a fall in both nasal volume and minimal cross sectional area. A secondary aim was to study if this could be prevented by humidifying the air inspired with nasal CPAP. METHODS Nasal CPAP was applied to eight normal subjects who kept their mouths open until they had expired 500 litres. The effect of this on nasal mucosal blood flux and nasal geometry was studied with and without humidification using a laser Doppler blood flowmeter and acoustic rhinometer. In addition, nasal mucosal blood flux was measured in four of the eight subjects before and after nasal CPAP with the mouth closed. RESULTS Nasal CPAP using room air with the mouth closed did not result in any change in nasal mucosal blood flux; with a mouth leak nasal CPAP using room air was associated with a 65% increase in nasal mucosal blood flux. There was no change in nasal geometry. Nasal CPAP using humidified air with a mouth leak did not cause any change in nasal mucosal blood flux or nasal geometry. CONCLUSIONS Nasal CPAP used with an open mouth leads to an increase in nasal mucosal blood flux. This can be prevented by humidifying the air inspired with nasal CPAP.