Increasing numbers of total hip arthroplasties in combination with increasing age and growing daily activities of the elderly lead to increasing numbers of periprosthetic fractures and revision arthroplasties in osteoporotic bone. The prosthesis nail is a hybrid of a hip prosthesis and an intramedullary nail allowing immediate full weight bearing and early rehabilitation. The prosthesis nail consists of three self-locking components: a distally locked intramedullary nail, different lengthening modules, and a hip prosthesis module. From 1992 to 1999, 28 prosthesis nails were implanted in 26 patients (40-88 years, mean age: 71 years). The indications were 21 peri- and subprosthetic fractures caused by trauma, 2 fractures of the proximal femur in combination with a fracture of the femoral head or severe degenerative arthritis, and 4 revision arthroplasties associated with poor bone quality. A combination of the prosthesis nail and bone cement was used in one patient suffering from a pathological fracture of the distal femur. Patients were additionally treated with wire cerclage (six patients) and autogenous bone grafting (ten patients). All patients-except those who received a reconstruction of the acetabulum in the same session (four patients)-were mobilized with full weight bearing on the operated side as soon as wound pain diminished. Bone healing was observed in all periprosthetic fractures. In three patients the prosthesis nail had to be revised: one patient suffered from recurrent dislocations and in one patient weighing 350 pounds limb shortening occurred after the distal locking screw broke. Intramedullary infection was observed once after treatment of a periprosthetic fracture. When bone union was achieved the prosthesis nail was removed and the patient was mobilized with a girdlestone situation. The idea of the prosthesis nail is based on the logical consequence of treating femur fractures with the most efficient procedure, which is intramedullary nailing. The prosthesis nail can be applied according to the requirements of the fracture as a reamed or unreamed nail and immediate full weight bearing is possible. Considering the high average age of the patients, low morbidity, short rehabilitation time, and low costs are the major advantages of this new device. Taking into account the unfavorable preoperative conditions associated with elderly and multimorbid patients, the rate of complications is relatively low.