Between 1980 and 1997, we treated 39 patients (mean age 39 (16-66) years, 24 men) with megaprosthetic replacement of a large bone defect (> 10 cm) of the pelvis. The bone resection was necessary in 38 cases due to malignant bone and soft tissue tumors and in 1 case due to hydatid disease. Polyacetal hemipelvic replacement was performed in 29 cases, CAD hemipelvic replacement in 8 cases, and a saddle prosthesis was inserted in 2 cases. All patients were followed clinically and radiographically. The mean follow-up was 58 (15-110) months. 20 patients have died of their tumor. 10 local infections occurred, in 2 cases necessitating hemipelvectomy. Hip dislocation occurred in 6 cases. 13 of the remaining 17 survivors had good or excellent clinical results according to the Enneking evaluation (MSTS). In 6 of the 17 survivors, radiographs revealed implant loosening.