The dispersion of an aerosol bolus in a model lung airway was analyzed numerically. Doubly bifurcated tube models were used in order to eliminate the sensitive effect of the inlet velocity profile on the entire flow field. Navier-Stokes equation and diffusion equation were solved by CFX-F3D, an FVM commercial code, and the broadening of concentration peak was converted into an effective diffusivity D-eff. The local D-eff for each generation could be expressed approximately as a linear function of local mean flow velocity and diameter, but secondary how makes the relationship nonlinear. It was found that D-eff increases toward downstream due to streaming, and that D-eff for particles is 35% lower than for gases. Simulation results were in good agreement with experiments by Scherer when the difference in diffusivity between gases and particles was corrected for. (C) 2000 Elsevier Science Ltd. All rights reserved.