The friction and wear of ultra-high molecular weight polyethylene (UHMWPE) pins sliding against a stainless steel disc were measured for sliding speeds ranging from 1.25 to 10 m s-1 and disc surface roughnesses R(a) from 0.07 to 0.53-mu-m ms-1. Frictional heating was controlled by air jets and surface temperature measured with an IR pyrometer. It was found that the wear of UHMWPE is critically dependent on surface temperature and that, when the temperature exceeds a critical value, wear proceeds in a series of discrete steps caused by the sudden loss of a molten or softened layer of polymer. Wear was also influenced by surface roughness. An optimum surface roughness, ie. a minimum of wear was found at low and medium sliding speeds. At the highest speed tested, however, the influence of roughness on wear rate was much less distinct. Scanning electron photomicrographs of worn pins and disc surfaces revealed evidence of melting by UHMWPE at high sliding speeds and abrasion at high surface roughnesses. Transfer films on disc surfaces were limited to isolated deposits of polymer wear particles.