Despite the fact that powder inhaler devices have been available for over 3 decades there is still very little fundamental information as to how powder inhaler design interacts with the airway dynamics of patients. This paper documents the results of human volunteer investigations designed to elucidate this interaction. These data indicates that if 'maximum' inhalation effort is employed the flow rate attained through an inhaler is controlled by the maximum pressure drop developed by the chest muscles. If a moderate or 'comfortable' effort is used, however, the relationship is more complex. An equation for defining the 'respirable fraction' of an inhalation aerosol cloud as a function of inhalation flow rate is also proposed.