This paper describes the use of a discrete particle computer simulation to test whether the apparent low friction exhibited by long runout landslides could be explained in terms of simple granular mechanics. The flow structure consists of an active basal shear zone supporting the majority of the landslide, which travels as a relatively solid plug. The observed runout appears to depend on the total energy dissipation which is a trade-off between (1) the inelasticity of the particles, which governs the energy dissipated in each event, and, (2), the size of the dissipating shear zone, which governs the number of particles that are actively dissipating energy. Very inelastic particles quickly dissipate the work performed at boundaries and result in thin dissipative zones. As a result, these two factors balance and the runout is nearly independent of particle inelasticity. Along the same lines, increasing the boundary roughness was found to increase the size of the dissipative shear zone and markedly decrease the runout. Also, a distribution of particle sizes destroyed ordered structures within the body which, once again, increases the dissipation and decreases the runout. Unfortunately, the simulations also indicate that the runout should be independent of the depth of the flow and thus cannot account for the observed volumetric effect on runout. ¿ American Geophysical Union 1993