The effect of air now and acceleration on the particle size distributions of two Turbuhalers containing drug loads of 0.5 and 1.3 mg per dose was determined with a time-of-flight aerosol beam spectrometer (Aerosizer(R)). While the particle size of both inhalers decreased with increasing now and acceleration, the distributions became narrower and variability was reduced. Simultaneously, a decrease in the number and mass of particles measured was observed which was more pronounced for the 1.3 mg dose. Additionally the count rate for the 1.3 mg dose was lower than for 0.5 mg. These observations were accompanied by a remarkably fine particle size distribution for the high dose Turbuhaler(R). It was concluded that the quantity of particles in the DPI aerosols exceeded the resolution of the Aerosizer, erroneously shifting the size distributions towards smaller diameters. In addition the DPI aerosols were analyzed with a Four Stage Impinger to evaluate the results. The particle size estimates obtained using the impinger were in the same range as those obtained at an acceleration of 5 L/s(2) using the Aerosizer to study the 0.5 mg Turbuhaler. The Aerosizer produced significantly smaller particle sizes than the impinger in all cases evaluating the 1.3 mg Turbuhaler, supporting the theory of distorted particle size distributions due to particle overload.