Persistent, semi-volatile organochlorine compounds, including toxic industrial pollutants and agricultural pesticides, are found everywhere on Earth, including in pristine polar and near-polar locations [1-4]. Higher than expected occurrences of these compounds in remote regions are the result of long-range transport in the atmosphere, precipitation and 'cold condensation'-the progressive volatilization in relatively warm locations and subsequent condensation in cooler environments [3,4] which leads to enhanced concentrations at high latitudes. The upper reaches of high mountains are similar to high-latitude regions in that they too are characterized by relatively low average temperatures, but the accumulation of organochlorine compounds as a function of altitude has not yet been documented. Here we report organochlorine deposition in snow from mountain ranges in western Canada that show a 10- to 100-fold increase between 770 and 3,100 m altitude. In the case of less-volatile compounds, the observed increase by a factor of 10 is simply due to a 10-fold increase in snowfall over the altitude range of the sampling sites. In the case of the more-volatile organochlorines, cold-condensation effects further enhance the concentration of these compounds with increasing altitude. These findings demonstrate that temperate-zone mountain regions, which tend to receive high levels of precipitation while being close to pollutant sources, are particularly susceptible to the accumulation of semivolatile organochlorine compounds.