The records of 11 consecutive adult patients who underwent revision of a failed femoral megaprosthesis (aseptic loosening, nine; periprosthetic fracture, one; and prosthesis fracture, one) to an allograft prosthesis composite were reviewed retrospectively. Complications included radiographic component subsidence in two patients (18%), hip instability in three patients (27%), deep infection in two patients (18%), and allograft fractures in two patients (18%). Five patients (45%) required subsequent reoperations; four patients underwent removal of the allograft prosthesis composite at a mean of 16 months (range, 5-41 months) and one patient (9%) with hip instability underwent revision of the failed megaprosthesis to a constrained acetabulum. Reconstruction of a failed femoral megaprosthesis is a complex problem caused by extensive bone loss and violation of soft tissue attachments. Despite a high complication rate, six patients (55%) remained ambulatory and had mild or no pain at a mean followup exceeding 5.5 years.