Acetabular erosion, intrusion of the prosthesis into the pelvis, is the most serious late complication of hemiarthroplasty. For the detailed observation on this lesion, the authors reviewed 55 patients who underwent femoral head replacement and were followed up to at least 1 year after operation. The acetabular erosion were present in 18 patients(32.7%) out of the total 55 patients. Based on the results of this study, the authors presented a new classification of acetabular eorsion: Stage I(erosion of the cartilage only) was seen in 11 patients, stage II (evidence of the initial bony erosion) in 4 patients,and stage III(intraosseous intrusion of the metallic head) in 3 patients. Revision arthroplasty was obligatory in two of the three stage III patients. The severity of symptoms did not always correspond with the radiological changes. The acetabular erosion was more common in younger patients, in male patients and in patients with avascular necrosis of femoral head than in older patients, in female patients, and in patients with femoral neck fracture. The tendency of acetabular erosion occurring mainly in younger and male patients is probably attributed to their better survival and greater activity.