Abstract: A stem for an endoprosthesis to be anchored in a bone (3) comprises a chamber (4) which surrounds the stem periprosthetically, which can be sealed in a pressure tight manner and which can be expanded with a thermoplastic or thermostable filling material (16). The rigid stem (1) of the endoprosthesis is connected mechanically to the bone (3) via the expandable chamber (4) by means of the filling material (16). The filling material is isolated from the bone (3), however, by the wall of the pressure-tight chamber (4). This makes it possible to ensure that the outer wall of the chamber (4) adjacent to the bone is extremely biocompatible.