The present invention relates to a system for providing respiratory therapy to a patient comprising:a patient interface (108) adapted to present a respiratory gas at an entrance to a patient's airways；an air delivery conduit (106) connected to the patient interface； andapparatus to provide a controllable supply of air having a flow rate, a temperature and a humidity, the apparatus being controlled to vary at least one of flow rate, temperature and humidity,the apparatus being adapted to provide a first level of humidification during an inspiratory portion of a respiratory cycle of a patient and a second level of humidification during an expiratory portion of the respiratory cycle.
