The success of Moore and Thompson endoprosthesis in arthroplasty of the hip during the past three decades has prompted their continued use with but few alterations. Recently, to reduce the intolerance of acetabular cartilage and bone to frictional shear forces, the conventional endoprosthesis was modified, so many types of bipolar arthroplasty were introduced and implanted. But, the movements of the two components and acetabular erosion are rather controversial still. Among the 51 cases of bipolar endoprosthesis those have been operated at department of orthopaedic surgery, National Medical Center from 1981 to 1986, we analysed 40 cases for acetabular erosion and 22 cases for motion study of two components under the fluoroscopy. Following results were obtained, 1. Among 40 cases, there was no true protrusio acetabuli and increased CE angle in 12 cases(30%), which were limited to cartilage. 2. Under the fluoroscopic study, we found motion at inner and outer bearing joint in 14 cases, inner bearing joint only in 6 cases and outer bearing only in 2 cases. 3. The CE angle was increased more in younger age group than older group. 4. According to indication of bipolar arthroplasty, CE angle was increased more in disease group than trauma group. 5. The CE angle was increased more in leg length discrepancy and head piece size discrepancy group than the other group. 6. The CE angle can be substituted for joint space measurement. 7. The CE angle was increased more in low Harris hip rating score group. 8. Harris hip rating score had no correlation to motion of two components.