Loss of acetabular bone renders secure fixation of an artificial cup difficult. Loss of pelvic bone mainly results from repeated revision of failed THR, tumor resection within the acetabular region and trauma. The main criteria for the method of treatment to be applied are the biomechanical condition of acetabular defects, the bacteriological situation, and the patient himself. In aseptic cases we regard bony reconstruction with homografts as the method of choice. From 1982 to 1986 about 1000 bony reconstructions were performed using more or less extensive facilities for support, i.e. screws, baskets, plates. In septic cases defects are closed using antibiotic-loaded acrylic cement in combination with screws. In the extreme case of bone destruction we recommend the saddle prosthesis as an alternative measure. This type of prosthesis was used in 76 cases from 1979 to 1984. Other methods of treatment, such as resection arthroplasty, arthrodesis, and partial pelvic replacement may be considered.