The aerodynamic size distributions of 9 commonly used bronchodilator aerosols were determined by using a new instrument, the single-particle, aerodynamic relaxation time analyzer, which can measure the aerodynamic size of single particles and droplets in real time independent of density, shape, and other physical characteristics. The following commercial preparations were studied: Duo-Medihaler, Medihaler-Epi, Medihaler-Epi Medihaler-Iso, Norisodrine Aerotrol, Alupent, Metaprel, Isuprel Mistometer, Bronkometer, and Asthma-Meter. Count median diameters ranged from 0.62 to 0.82 micrometer, and mass median aerodynamic diameters ranged from 2.8 to 4.3 micrometer. The heterodisperse nature of the aerosols was demonstrated by the relatively large geometric standard deviations, which ranged from 1.5 to 2.1. None of these aerosols had a log-normal distribution; rather, the distribution curves were multimodal.