A metered dose inhaler is provided for inhalation of asthmatic medication. An upright cylindrical air chamber has structure at the top for receiving a pressurized canister of asthmatic medication and for conducting metered doses thereof into the air chamber. A one way valve is provided at the bottom of the air chamber to admit air but to preclude escape of mist. A mouthpipe is provided at the top of the chamber on one side thereof for insertion in a patient's mouth. A deflectable diaphragm is provided in cooperation with the mouthpipe so that when a patient inhales the diaphragm deflects to permit the mist in the air chamber to be inhaled, air entering the one way valve of the bottom of the air chamber to supplant the mist. When the patient exhales the diaphragm closes, and the exhaled air is bypassed to the outside.