A study of microhardness at the wear surface of polyethylene implants before and after use is reported. The microhardness values support previous electron microscopic observations of an embrittled region extending into used implants by showing that, after use, the hardness of polyethylene implants increases above the initial value the nearer the wear surface is approached. It is shown that these changes accompany a hardening of the surface layer and are related to an increase in the crystallinity of the polymer. The increasing hardness towards the edge of the concave surface of the implant is discussed in tl:rms both of the local variation of crystallinity and changes in the interlamellar regions.