A pressure support system for treating lung disease, comprising: a pressure generating system (52, 60) adapted to produce a flow of breathing gas; a patient circuit (56, 58) operatively coupled to the pressure generating system to deliver the flow of breathing gas to an airway of a patient; and a controller operatively coupled to the pressure generating system to control the pressure generating system to deliver the flow of breathing gas to the patient at a pressure level during at least a portion of an inspiratory phase of the patient, the pressure support system being characterized in that the controller is adapted to determine a lung compliance or upper airway resistance indicative of the patient's pulmonary mechanics, the controller being arranged to control the pressure generating system to determine and vary the pressure level based on the determined lung compliance or upper airway resistance by (a) delivering a first bi-level positive pressure support at an IPAP/EPAP differential for a first number of respiratory cycles, the IPAP/EPAP differential of the first bi-level positive pressure support being a baseline level based on a predetermined baseline IPAP level and a predetermined baseline EPAP level; (b) determining the lung compliance or upper airway resistance; (c) determining that the lung compliance or upper airway resistance is outside of a predetermined baseline range; (d) increasing the IPAP/EPAP differential to a current IPAP/EPAP differential; (e) delivering a second bi-level positive pressure support at the then current IPAP/EPAP differential for a second number of respiratory cycles; (f) re-determining the lung compliance or upper airway resistance; and (g) repeating (d), (e) and (f) one or more times until either it is determined that the then current re-determined lung compliance or upper airway resistance is within the baseline range or that the then current IPAP/EPAP differential is equal to a predetermined maximum differential. a pressure generating system (52, 60) adapted to produce a flow of breathing gas; a patient circuit (56, 58) operatively coupled to the pressure generating system to deliver the flow of breathing gas to an airway of a patient; and a controller operatively coupled to the pressure generating system to control the pressure generating system to deliver the flow of breathing gas to the patient at a pressure level during at least a portion of an inspiratory phase of the patient, the pressure support system being characterized in that the controller is adapted to determine a lung compliance or upper airway resistance indicative of the patient's pulmonary mechanics, the controller being arranged to control the pressure generating system to determine and vary the pressure level based on the determined lung compliance or upper airway resistance by (a) delivering a first bi-level positive pressure support at an IPAP/EPAP differential for a first number of respiratory cycles, the IPAP/EPAP differential of the first bi-level positive pressure support being a baseline level based on a predetermined baseline IPAP level and a predetermined baseline EPAP level; (b) determining the lung compliance or upper airway resistance; (c) determining that the lung compliance or upper airway resistance is outside of a predetermined baseline range; (d) increasing the IPAP/EPAP differential to a current IPAP/EPAP differential; (e) delivering a second bi-level positive pressure support at the then current IPAP/EPAP differential for a second number of respiratory cycles; (f) re-determining the lung compliance or upper airway resistance; and (g) repeating (d), (e) and (f) one or more times until either it is determined that the then current re-determined lung compliance or upper airway resistance is within the baseline range or that the then current IPAP/EPAP differential is equal to a predetermined maximum differential. (a) delivering a first bi-level positive pressure support at an IPAP/EPAP differential for a first number of respiratory cycles, the IPAP/EPAP differential of the first bi-level positive pressure support being a baseline level based on a predetermined baseline IPAP level and a predetermined baseline EPAP level; (b) determining the lung compliance or upper airway resistance; (c) determining that the lung compliance or upper airway resistance is outside of a predetermined baseline range; (d) increasing the IPAP/EPAP differential to a current IPAP/EPAP differential; (e) delivering a second bi-level positive pressure support at the then current IPAP/EPAP differential for a second number of respiratory cycles; (f) re-determining the lung compliance or upper airway resistance; and (g) repeating (d), (e) and (f) one or more times until either it is determined that the then current re-determined lung compliance or upper airway resistance is within the baseline range or that the then current IPAP/EPAP differential is equal to a predetermined maximum differential.