Abstract: An intramedullary nail for the lengthening of bones. It comprises base and extension means and adjustment and rectilinear means. The base means includes an elongated means having a longitudinal axis, and axial opening therethrough and having an inner surface, and distal and proximal ends. The adjustment means has an axis, an outer surface, and is disposed within the distal end of the base means. The rectilinear means are formed on the inner and outer surfaces of the axial opening of the base means and the adjustment means, respectively, for permitting rectilinear motion of the adjustment means toward the proximal end of the base means. The extension means has an axis, and is telescopically disposed within the proximal end of the base means; it is affixed to the adjustment means.