A non-dimensional functional relationship was constructed to characterize the fine particle fraction produced by dry powder inhalers. It was based on the values of steady-state pressure drop, turbulent kinetic energy produced, by the nozzle, as well the aerodynamic behavior of the powder particles as characterized by their modes of the size distributions. Dimensional analysis was used together with physical reasoning to formulate the functional and reduce the number of dimensionless parameters. Various combinations of inhaler devices, analytes and air flow rates have been evaluated with respect to basic particle characteristics, aerosol deposition behavior, viscous pressure drop and air flow turbulence. The resulting experimental data were employed to find the approximate dependence between the four non-dimensional groups retained in the functional and a very good correlation has been achieved. As a conclusion, the size of the particles has been identified as the most relevant variable, followed by the viscous pressure drop in the inhaler device and the density of the powder. The present results may be used to improve the design of powder-device combinations. (c) 2007 Elsevier Ltd. All rights reserved.