Liquefied gases, such as chlorine and ammonia, are stored in large quantities at industrial sites. If released accidentally, they form a heavy gas cloud that has the potential to kill or injure large numbers of people. The dispersion of such a cloud is thus of interest to the risk assessment community [Nussey, Pantony, & Smallwood, 1992. HSE's risk assessment tool, RISKAT. In: Major Hazards: Onshore and Offshore. pp. 607-638]. Little is understood about the effect of slope on risk. Here, the risk (probability) of being exposed to the gas cloud, given a release, is considered: probability language is needed because wind direction is assumed to be a random variable. This paper shows how the risk of being exposed to toxic gas released over a slope may be estimated using simple physical modelling. The physical model used is that of Tickle [J. Hazard. Mater. 49 (1996) 29], who showed that a finite-volume instantaneous release on an inclined plane can form a stable wedge-shaped cloud that moves down the line of greatest slope. Nonzero windspeeds are accounted for by following Tickle's suggestion of vectorially adding windspeed to the advection induced by the slope. A range of windspeeds and slopes are considered. The slopes substantially affect the risk in the sense that the predicted risk contours are far from circularly symmetric. (C) 2003 Elsevier Science Ltd. All rights reserved.