Particle deposition in human lungs was investigated theoretically considering a stochastic variation of airway morphology using a Monte Carlo method. In computing the total and regional deposition each airway generation was divided into infinitesimal volume segments and each volume segment was allowed to pass through randomly selected airway branches of which morphology (e.g., airway dimensions and branching angle) was varied randomly. Deposition values obtained by the Monte Carlo method were compared with those obtained by the traditional deterministic method. It was found that the Monte Carlo method predicted deposition values generally comparable to those predicted by deterministic method for sub-micron size particles. However, for micron size particles the Monte Carlo method provided greater deposition values in the proximal airway regions where deposition occurs mainly by inertial impaction. The difference could be attributed to the non-linear relationship between deposition efficiency and airway dimensions in the inertial deposition regime. The results suggest that a Monte Carlo method may be a useful tool for evaluating deposition of inhaled particles in the lungs with a wide variation of airway dimensions. Published by Elsevier Science Ltd.