Mass transport phenomena in airways, including dispersion, may be influenced by the complicated three-dimensional (3D) branching geometry, which undergoes expansion and contraction during the respiratory cycle. In this study, we investigated the effects of this expansion and contraction on gas dispersion in multi-branching airways using computational fluid dynamics. A 3D multi-branching airway model was constructed based on mammalian computed tomography images, with diameters ranging from 1.25 mm in the parent tube to 0.35 mm in the daughter tube. We examined the dispersion of oxygen in the airway when subject to oscillatory flow while the airway expanded and contracted sinusoidally with time. After several respiratory cycles, the oxygen fraction was higher in the airway-motion model than in the rigid model, and it increased as the volume expansion factor increased. Furthermore, the oxygen fraction increased as the breathing frequency was increased for the airway model undergoing wall motion. Transport simulation of passive tracer particles showed that, although the particles were dispersed around the initial position in the rigid model, many particles were anchored around the carina and dispersed widely in the airway-motion model. These results indicate that steady streaming and unsteady flow behavior due to airway branches and expanding and contracting motion may enhance gas dispersion in multi-branching airways. (C) 2013 Elsevier Ltd. All rights reserved.