THE mobility of pyroclastic flows has been described as spectacular1, in particular because of their ability to surmount obstacles in their path. It is relatively easy to estimate the minimum velocities required to climb a given height, if frictional losses are ignored. We have used data on well documented largecold rock avalanches to estimate the magnitude of frictional losses, and used these estimates to calculate velocities for some large pyroclastic flows. We argue that frictional losses in large pyroclastic flows must be comparable with, or less than, those in the most mobile rock avalanches, and calculate from data on heights climbed that velocities of large pyroclastic flows may be very high, up to 140 m s−1. We also conclude that the height of the eruption column giving rise to a large pyroclastic flow need only be a few hundred metres in some cases.