To establish the types of wear particles and associated tissue response around human prostheses the periprosthetic tissues around 50 hip arthroplasties revised for aseptic loosening of one or more components were examined by light microscopy, transmission electron microscopy, and energy-dispersing x-ray microanalysis. The tissues around cementless metal-on-bone and ceramic-on-ceramic prostheses contained few or no prosthesis wear particles. The tissues around metal-on-metal prostheses contained large numbers of metal particles and large numbers of macrophages, and occasional multinucleate giant cells. The tissues around cemented metal-on-polyethylene prostheses often contained large numbers of small and large polyethylene particles, variable numbers of cement particles, and occasional metal particles. Large numbers of macrophages and multinucleate giant cells were frequently seen in these tissues. Lymphocytes were occasionally seen in association with metal particles. Ultrastructural studies of the periprosthetic tissues confirmed the phagocytosis of submicroscopic wear particles by macrophages. Varying degrees of degenerative change in macrophages were seen in association with phagocytosis of metal particles. Large numbers of cytolysosomes were seen in cells in association with the accumulation of wear particles.