This paper presents regional and total deposition measurements in several realistic mouth and throat geometries and demonstrates the effects of intrasubject and intersubject variations in geometry on deposition efficiency. A set of seven geometries that span the range of key dimensions of a larger set of 80 geometries were studied at flow rates of 30 and 90 l/min and particle diameters of 3-6.5 mum. The models were obtained using magnetic resonance imaging scans. Gamma scintigraphy and gravimetry were used to determine deposition. The models were subdivided into regions corresponding to the mouth, oropharynx, larynx and trachea. The results indicate that both total and regional deposition exhibit large intersubject differences, but also show some intrasubject difference. A mean equivalent diameter D-mean=2 rootV/piL (where V is mouth-throat volume, L is centerline path length), and corresponding velocity U-mean = 4Q/piD(mean)(2) (where Q is inhalation flow rate) are used as the length and velocity scale in defining a Stokes number, Stk. In addition, an effect on deposition due to flow Reynolds number, Re was noticed. When plotted vs. Stokes number with inclusion of this Reynolds number effect, the total deposition data from all the different geometries collapse onto essentially a single curve, given by: eta = 100-100/(11.5 Stk(1.912) Re-0.707 + 1) for the parameter range explored, where eta is the percent of inhaled aerosol depositing in the mouth-throat. (C) 2004 Elsevier Ltd. All rights reserved.