Abstract: A device for elongating long bones such as thigh bones, shin bones and arm bones, including an intramedullary nail with a tubular sleeve (4), and an extension (5) axially slidable within the sleeve (4). The sleeve and the extension include means (6; 7) for rigidly connecting them to two bone portions separated by an osteotomy, and means for moving the extension (5) relative to the sleeve (4), whereby both the intramedullary nail and the corresponding bone may gradually be elongated. The means for moving the extension (5) relative to the sleeve (4) include an electric motor (8) arranged within the sleeve (4) and linked to a speed reducer (9) driving a screw/nut assembly (13, 14, 15) for moving the extension (5) relative to the sleeve, and means (2) for supplying power to the electric motor (8) and automatically controlling the value and direction of the movement imparted to the extension (5) in the sleeve (4) by the screw/nut assembly (13, 14, 15) driven by the electric motor (8).