The aerodynamic size distribution of several bronchodilator and corticosteroid metered-dose inhaler (MDI) aerosols was estimated in both dry and humid (90% RH) air environments with a six-stage cascade impactor. The distribution of aerosol size that penetrated into a simulated lung model were also measured. The size distributions were approximately log-normal and ranged from 2.4 to 5.5 micron in mass median aerodynamic diameter (MMAD) with geometric standard deviation (GSD) of 1.7 to 2.5 in a dry environment. In humid air, MMAD increased from 1 to 26% above the dry air state, but GSD remained unchanged. The size of aerosol delivered by MDI that penetrated into a simulated lung model fell to 2.4 to 2.8 micron in MMAD (GSD, 1.9 to 2.2). In contrast to aerosols produced by MDI, MMAD of an aerosol of cromolyn sodium powder dispersed by a Spinhaler increased rapidly with increasing humidity, 5.6 +/- 0.3 micron in dry air and 10.1 +/- 0.8 micron in 90% RH air. Finally, the factors influencing size of MDI-delivered aerosols, including formulation, canister pressure, physicochemical properties of propellants, and design of the valve and actuator orifices are discussed. Effective delivery of MDI-generated aerosols into the lung is highly dependent on particle dynamics and jet flow, and no single parameter can produce a unique particle size and jet pattern.