A patient interface device uses a pressure flap which extends inwardly from an outer edge of a mask of the device. The pressure flap is for contacting the skin of the patient to provide a seal or partial seal between the mask volume and the ambient surroundings. The pressure flap comprises a cavity area which is exposed to a reduced pressure compared to the mask volume. This assists in keeping the pressure flap pressed against the patient's skin, and thereby maintain a large contact area. This in turn gives a reduction in local high pressure areas.