Aims: to evaluated the radiological evolution of autologous bone graft in acetabular structural and cavitary defects in primary total hip artrhoplasty using an expansion cup (CLS, Sulzer Orthopaedics). Methods: we reviewed, retrospectively, 19 patients (9 males, 10 females, average age 64,7 years) treated in our Unit from January 1994 through July 2000 with acetabular bone grafting. According to the AAOS classiþcation, 11 patients showed a segmental acetabular deþciency requiring a structural graft, 8 had a cavitary defect treated with avoid-þlling bone. X-rays were standardized and investigated for evidence of graft remodelling (obliteration of the interface bone-graft, change in density, bridging trabeculation, reorientation of trabecular pattern) and cupñs loosening (radiolucency, cup migration, ratio graft/cup, breakage of screws). Results: all, but one, patients were evaluated at aminimal follow-up of 2 years (average time 53 month). Osteo-integration was appreciated radiographically at 6 months while union at 12 months in all cases. No cupñs loosening was experienced. Reasorption was evident as a round off and smoothing of the edge of the structural graft in 8 patients while the percentage of bone graft surrounding the cup did not change over time. Conclusions: we believe that the use of an expansion cup, with its elastic properties, allows micro-movements at the interface bone-implant creating a dynamic system that could improve osteo-induction and osteo-integration of the graft.