Abstract: Apparatus for measuring the respiratory resistance of a patient has a passageway for the flow of air produced by the patient's exhalation and a valve for temporarily halting and then restoring the flow. The rate of flow following the temporary halting depends solely upon the respiratory resistance. Respiratory resistance is derived from circuitry responsive to signals representing pressure and rate of flow. The rate of flow is preferably derived from a comparison of the pressure within the apparatus with the ambient pressure. Provision may be made for actuating the valve automatically when satisfactory conditions are established, for visual display of the resistance and for averaging the resistance results over a number of exhalations.