We studied the effect of gravitational sedimentation on the dispersion of 0.5 and 1μm-diameter particle boluses within a two-dimensional symmetric six-generation model of the human acinus. Boluses were introduced at the beginning of a 2-s inspiration immediately followed by a 4-s expiration, in normal gravity (1G) and in the absence of gravity (0G). The flow corresponded to a flow rate at the mouth of 500ml/s. In 0G, simulated dispersion (H s i m ) was 16ml for both particle sizes. In 1G, H s i m was 71 and 242ml for 0.5 and 1μm-diameter particles, respectively, showing the effect of gravitational sedimentation. The difference between experimental data (J. Appl. Physiol. 86 (1999) 1402) and simulations was independent of particle size. This suggests that the residual dispersion was independent of the intrinsic properties of the particles and was more likely due to other mechanisms such as ventilation inhomogeneities, cardiogenic oscillations and alveolar wall motion.