The efficacy of airborne pharmaceuticals in the treatment of lung diseases may be improved with the selective deposition of inhaled drugs. Herein, a validated mathematical model is used to examine the effects of aerosol polydispersity upon deposition in the human lung. Localized deposition patterns are calculated on an airway-by-airway basis. For log-normal particle size distributions, as produced by metered-dose inhalers (MDIs) and nebulizers, deposition efficiencies are shown to be sensitive functions of the mass median aerodynamic diameter (MMAD) and geometric standard deviation (GSD). The relevance of the count median diameter (CMD) of a log-normal distribution is also examined. The model provides a heretofore unavailable basis for interpreting and information obtained from gamma camera lung scans. The agreement between theory and experiment indicates that the behavior of inhaled particles is accurately modeled.