Most theoretical predictions of the rate of Brownian-induced coagulation have considered the continuum hydrodynamic resistance of the suspending gas or liquid as well as van der Waals and/or electrostatic forces. However, the mean-free path of the gas is comparable with the length scale at which van der Waals forces become important during the collision of two Brownian aerosol particles. We predict the initial rate of coagulation in a monodisperse aerosol driven by Brownian motion in the presence of van der Waals forces and non-continuum lubrication forces when the Knudsen number (ratio of the mean-free path to the particle radius) is small but non-zero. A comparison with experiments demonstrates the accuracy of the predictions and the importance of accounting for non-continuum effects to provide quantitative predictions of coagulation rates. (C) 2005 Elsevier Ltd. All rights reserved.