Investigations of the friction and wear of Ultra-high Molecular Weight Polyethylene (UHMWPE) in reciprocating sliding against a range of stainless steel counterfaces of various initial surface roughness, are described in this paper. The tests were carried out either with the wear specimen under dry conditions of contact or lubricated by distilled water. A single sliding speed of 0.097 m/s and two constant loade of 5N and lON were used. The results have shown that there is an optimum scale of surface roughness which yields minimum dry wear rate and coefficient of friction for UHMWPE sliding on EN 58J stainless steel. The tests conducted under water lubricated conditions showed an appreciable reduction in the coefficients of friction and wear rates compared with the values obtained under bry conditions. A mechanism is proposed for the increased wear rate and coefficient of friction on smoother surfaces.