We reviewed the cases of fourteen patients (fifteen lesions) who had osteolysis following the replacement of the acetabulum without cement. Nine women and five men, seventeen to sixty-seven years old, were involved in the study. One woman had bilateral pelvic osteolysis. Eight of the fifteen index acetabular reconstructions were done with a titanium-alloy implant and seven, with a chromium-cobalt-alloy implant. Eleven of the fifteen acetabular components had holes in the metal shell that may have acted as a conduit through which wear debris could gain access to the implant-bone interface, but only two of the acetabular components had been fixed with screws. In these two acetabular components, all available screw holes were not filled. The polyethylene liner was eight millimeters thick or less in twelve of the fifteen acetabular components; all of the liners were ten millimeters thick or less. The diameter of the head of eleven of the fifteen femoral components was thirty-two millimeters. Fourteen of the fifteen femoral components were placed without cement, and all but one was radiographically stable. The duration from the index operation to the appearance of pelvic osteolysis ranged from fifty-three to eighty-four months (mean, sixty-five months). At the time of the diagnosis, the patients were functioning well clinically, and all but three had a Harris hip score of 90 points or better, despite extensive destruction of bone in some instances. Since these patients were functioning well, the pelvic osteolysis was diagnosed radiographically at a regular follow-up examination. Only one patient had evidence of migration of the acetabular component on serial radiographs.(ABSTRACT TRUNCATED AT 250 WORDS)