In order to compare the effects of using helium-oxygen and air in assisted breathing and inhalation therapies, flow and particle deposition results were obtained in a realistic model of human oral extrathoracic (ET) airways using computational fluid dynamics (CFD) and pressure loss measurements. As the main deposition mechanism for pharmaceutical aerosols in the ET is inertial impaction, the ET model was reconstructed from medical images to take into account the complexity of realistic morphological features. Calculations were performed with the CFD software Fluent (R), and pressure losses were measured on a cast based on a stereolithographic fabrication of the model. Results show that ET pressure loss and particle deposition are lower with helium-oxygen as compared to air. Moreover, further simulations were performed with various particle sizes and inspiratory flow rates, which indicate that particle deposition in the ET depends on both the Stokes and Reynolds number. (c) 2009 Elsevier Ltd. All rights reserved.