A system for diagnosis and treatment of breathing disorders in a patient, comprises a flow generator supplying an air flow to an airway of a patient via a flow path, a venting arrangement moveable between (i) a closed position in which the flow path is substantially sealed between the flow generator and the patient's airway and (ii) an open position in which the flow path is open to an ambient atmosphere, a sensor detecting data corresponding to flow through the patient's airway, and a processing arrangement controlling operation of the venting arrangement and the flow generator, wherein, in a diagnostic mode, the processing arrangement maintains the venting arrangement in the open position and in a therapeutic mode, the processing arrangement maintains the venting arrangement in the closed position and controls the flow generator to supply to the patient's airway via the flow path a calculated therapeutic pressure.