Epidemiology data show that mortality rates for chronic obstructive pulmonary disease (COPD) patients increase with an increase in concentration of ambient particulate matter (PM). This is not seen for normal subjects. Therefore, the U.S. Environmental Protection Agency (EPA) has identified COPD patients as a susceptible subpopulation to be considered in regulatory standards. In the present study, a computer model was used to calculate deposition fractions of PM within the lungs of COPD patients. The morphology of COPD lungs was characterized by two distinct components: obstruction of airways (chronic bronchitis component), and degeneration of alveolar structure (emphysema component). The chronic bronchitis component was modeled by reducing airway diameters using airway resistance measurements in vivo, and the emphysema component was modeled by increasing alveolar volumes. Calculated results were compared with experimental data obtained from COPD patients for controlled breathing trials (tidal volume of 500 ml, respiratory time of 1 s) with a particle size of 1 microm. The model successfully depicts PM deposition patterns and their dependence on the severity of disease. The findings indicate that airway obstructions are the main cause for increased deposition in the COPD lung.