The flow in a set of four realistic mouth-throat geometries at a flow rate of 30 L/min is studied in order to determine the effect of intrasubject and intersubject variations on the mean flow patterns and the turbulence fluctuations. Direct numerical simulations (DNS) are performed, which fully resolve all the scales in the flow, without requiring a turbulence model. An immersed boundary method is applied on curvilinear grids which simplifies the task of grid generation for the complex extrathoracic geometries and allows the use of a structured grid solver which increases the efficiency of the numerical scheme. Inspection of the mean, responsible for the convective transport of particles, and the fluctuating component of velocity, responsible for turbulent dispersion, allows us to explain in vitro deposition data in the literature obtained in the same mouth-throat models. The results provide insight as to how geometric variation affects aerosol deposition and explain the scatter in deposition data observed in the literature. Geometric variation is shown to have a large impact on both the mean velocity profiles and the turbulence intensities. Examination of the flow fields in the various mouth-throat geometries allows us to address the origin of the dependence of deposition on Reynolds number, and provide the physical significance of the empirical Reynolds number correction previously proposed in the literature. (C) 2012 Elsevier Ltd. All rights reserved.