Airway monitoring apparatus 30 includes a driver 6 for applying sound in the audible frequency range to the airway of a subject and a detector 7 for monitoring the response of the airway, e.g. by detecting transmitted sound signal components and/or reflected sound signal components. Variations in the detected sound signal, e.g. energy, due to attenuation of the signal, give an indication of the state of the airway, e.g. airway patency, and can be used to monitor sleep-disordered breathing events, such as apnea and hypopnea, and to provide a breathing event index, e.g. AHI. The apparatus may provide servo-control for a respiratory assist device, such as a positive airway pressure device, so as to provide an appropriate pressure level setting. It may allow for home use of such devices and for home titration. It may also assist in discriminating central and obstructive breathing events, and in providing a measure of airway resistance.