A method and system for measuring pulmonary function. The method comprises the following steps of: (1) providing or obtaining a measurement of a subject breathing an inert gas composition; (2) providing or obtaining a measurement of the subject breathing a replacement gas composition; and (3) extracting parameters from these two measurements. The system comprises: (a) a gas supply; (b) a subject interface; (c) an inert gas concentration measuring device, wherein the system; and (d) means for extracting parameters from the measurements by the inert gas concentration measuring device: (1) obtains a measurement with the measuring device through the subject interface of a subject breathing an inert gas composition supplied from the gas supply; (2) obtains a measurement with the measuring device through the subject interface of the subject breathing a replacement gas composition supplied from the gas supply; and (3) extracts parameters with the parameter extracting means from these two measurements.