Eighteen patients being treated for revision total hip arthroplasty for aseptic loosening an average of 9.5 years after primary cemented hip arthroplasty were studied. The patients were found to have been symptomatic an average of one year before revision. Review of the roentgenograms at the time of revision revealed that most patients had moderate to severe loosening of the prostheses by Charnley's criteria. These findings were confirmed at surgery. Study of the tissue surrounding the prostheses suggests that the prominent particles were methylmethacrylate "pearls" caused by cement fragmentation. These particles were surrounded by histiocytes that were actively synthesizing and releasing protein. Cement fragmentation and not polyethylene wear may be the major cause of late aseptic loosening of cemented hip prostheses.