The decrease of bone density after fracture is a well-known phenomenon. Several studies of the changes of bone density after fracture demonstrated bone density decreased not only at the fracture site, b ut also along the whole fractured bone. Being treated with locked intramedullary (IM) nailing, 15 patients with the long bone fractures of lower limbs were examined by Quantitative Computed Tomography (QCT) or Dual-Energy X-ray Bone Densitometer (Lunar DPX-L) a few days after nail removal. In the cases of QCT (5 patients of the tibia shaft fracture), cortical bone density, cortical thickness and geometrical shape (i.e. external antero-posterior and medio-lateral diameters) of the fracture site were compared to those of the contralateral side. On the other hand, in the case of Lunar' (5 patients of the tibia shaft fracture & 5 patients of the femur shaft fracture), only cortical bone density was compared to that of the contralateral side. The results of this study were as follows: 1. When treated by the locked intramedullary nailing of the long bone fractures, the bone density of the tibia & femur shaft fractures measured from the fracture site, proximal and distal sites to mid-fracture decreased. 2. There was a difference in the bone density of the uninjured long bone diaphysis of the tibia & femur shaft fractures. That is, in the case of the tibia shaft fracture, the bone density of the ipsilateral femur diaphysis measured by QCT increased 2 %, and increased 7 % when measured by Lunar. But, in the case of the femur shaft fracture, the bone density of the ipsilateral tibia diaphysis measured by Lunar decreased 4%. 3. In the case of the tibia shaft fracture, the cortical thickness in the fracture site measured by QCT increased 72 %, and external antero-posterior and medio-lateral diameters increased 7 % and 63 %. It seemed to be the formation of peripheral callus due to medullary reaming preceding intramedullary nail insertion. Therefore, this clinical study demonstrated that the mechanical property of the long bone fractures was restored and restricted weight bearing hardly needs to recommended when the nail was removed after the fracture union. 4. The bone densities of the tibia shaft fratures measured by QCT were not different with those measured by Lunar.