The invention relates to continuous positive airway pressure (CPAP) respiratory therapy apparatus for treatment of apnea, hypopnea and other sleep disorders, and particularly to a standby control for automatically operating an air blower in CPAP apparatus. A blower is connected through a hose and a mask, such as a nasal mask or a nasal cannula, to the patient's respiratory airway. Initially, the blower is in a standby mode in which it is either turned off or, preferably, operated at a low speed. When a patient starts breathing through the mask, the blower is automatically switched to an operating mode. In the operating mode, the blower may be controlled to initially supply a low air pressure to the mask which either is increased after a sufficient time delay to allow the patient to fall asleep or is increased in response to the detection of a respiratory event such as apnea or hypopnea. If patient breathing through the mask ceases because the mask has been removed or otherwise is no longer sufficiently sealed to the patient's airway, the blower is automatically returned to the low pressure standby mode. If the patient does not begin breathing through the mask within a predetermined time, the blower is stopped. An optional recorder measures the total time that the patient is breathing through the mask or the total time that the blower is in the operating mode to provide the prescribing physician or technician with patient compliance information.