The histological appearance of the implant-bone boundary in eight preparations from seven deceased patients in whom a polyethylene acetabulum socket had been inserted without cement between 3 weeks and 3 years previously is described. In all cases, a good or excellent clinical result had been present up to death. All sockets were firmly anchored macroscopically in the bone of the acetabulum. Three zones were defined in accordance with the mechanical strain on the surface of the cup: (a) zone of compression; (b) zone of transition in which mainly shear forces act; and (c) zone of decompression, which corresponds to the region below the "equator." It was noticeable that a layer of connective tissue separated the implant from the bone over almost the entire surface. This layer was very thin in the region of the pressure transfer layer, and thicker in the decompression zone. At sites at which pressure and shear forces act a fibrocartilaginous tissue was found, and the "subchondral" bone was only occasionally in direct contact with the polyethylene surface. At some sites, but above all in the region of the compression zone, very tiny particles that were birefringent in polarized light were found. Either these derive from very fine grindings from the prosthesis surface or they are to be interpreted as a result of "biodegradation" of the polyethylene. This observation permits the conclusion that the biocompatibility of the implant can be improved further by coating the polyethylene surface.