An apparatus for delivering medicinal aerosol on-demand during patient inhalation wherein delivery of aerosol from a nebulizer (22) is optimized by synchronizing delivery to the inspiration of a patient (58)is disclosed. A control unit (32) monitors airway pressure and controls the flow of gas so that aerosol delivery is triggered by patient inhalation, in which event delivery continues for a predetermined time period which is adjustable by the user. The apparatus can be used with a mouthpiece (56), a face mask (66) without an integral exhalation valve, or a face mask (68) with an integral exhalation valve. The apparatus significantly increases the efficiency of delivery of medicinal aerosol by sensing a patient's inhalation and exhalation cycles and delivering aerosol only after a certain inhalation pressure is sensed following a certain previous exhalation pressure. These pressures, which are compared against predetermined thresholds, correspond to the actual patient breathing condition. By monitoring the patient's inhalation and exhalation in this manner, aerosol delivery is synchronized with the patient's breathing pattern. In this way, the apparatus prevents aerosol contamination of the environment without requiring an exhalation filter since aerosol is delivered to the patient only during inhalation. Exhalation is not obstructed since aerosol is not delivered during exhalation. Further, when it is desirable to use an exhalation filter to reduce the amount of exhalation gas discharged into the environment, the apparatus reduces occlusion occurring in the exhalation filter.