Nanocrystalline solids, in which the grain size is in the nanometre range, often have technologically interesting properties such as increased hardness and ductility. Nanocrystalline metals can be produced in several ways, among the most common of which are high-pressure compaction of nanometre-sized clusters and high-energy ball-milling(1-4). The result is a polycrystalline metal with the grains randomly orientated. The hardness and yield stress of the material typically increase with decreasing grain size, a phenomenon known as the Hall-Fetch effect(5,6). Here we present computer simulations of the deformation of nanocrystalline copper, which show a softening with grain size (a reverse Hall-Petch effect(3,7)) for the smallest sizes. Most of the plastic deformation is due to a large number of small ＇sliding＇ events of atomic planes at the grain boundaries, with only a minor part being caused by dislocation activity in the grains; the softening that we see at small grain sizes is therefore due to the larger fraction of atoms at grain boundaries. This softening-will ultimately impose a limit on how strong nanocrystalline metals may become.