A system for treating a patient suffering from obstructive sleep apnea may include a mask, a portable air flow generator configured to generate air flow at a relatively low flow rate, and a tube connecting the air flow generator and the mask such that air flow from the generator passes through an air flow generator valve on the mask. The mask may include a contact surface for forming a seal between the mask and the patient's face such that the mask surrounds the patient's nostrils, an expiration valve that opens during expiration, and an air flow generator valve that opens during inspiration. In some embodiments, the mask may further include an inspiration valve that opens during inspiration to allow air from outside the system to enter the mask.