Abstract: A device for monitoring the patient's ability to exhale has a piston slidable within a body part by air exhaled through a mouthpiece against a spring. A vent allows part of the air to discharge between the mouthpiece and the piston. By providing one or more discharge openings having removable seals, the amount of air discharged can be set to suit particular circumstances. Advantageously there is provided an indicator movable by, but not returnable with, the piston, to show the maximum position on a scale. The surface of the body part adjacent the scale may be so surfaced as to receive written matter.