The mass of avalanches was investigated using two series of observations made at Rogers Pass in Western Canada. Large avalanches occur once in 3 – 25 years at each avalanche path, and their mass greatly exceeds that of annual maximum avalanches. Standard extreme-value frequency distributions cannot be applied. An empirical predictive equation for the mass of large avalanches is presented that takes into account the 30-year maximum water equivalent of the snow in the avalanche path, surface area of the catchment, and incline of the starting zone. Observed avalanches with an apparent return interval of 100 years or greater had masses close to the predicted values. Key words: snow avalanches, mass, frequency, design.