The effect of ultrasound energy on bone has been studied for a long time. In particular, multiple effects of low-intensity ultrasound energy have recently been demonstrated experimentally, such as increases in bending strength of fracture callus, acceleration of soft callus formation and endochondral ossification of the callus at the fracture site, stimulation of aggrecan gene expression, or modulation of TGF-beta synthesis and increase of calcium uptake. Clinically, prospective, randomized, and double-blind trials showed the efficacy of low-intensity ultrasound beam stimulation in the acceleration of fracture healing, with a significant decrease in the time to healing. On the other hand, callotasis, a popular method for bone lengthening, requires much time for new bone formation, and an external fixator must be remain on the patient for a long period. This is one of the major problems of the callotasis technique. If ultrasound energy stimulation could accelerate the rate of callus formation in callotasis, the external fixator could be removed earlier, the treatment period could be shortened, and the patient could return to daily activities more quickly. We report on the use low-intensity ultrasound beam stimulation during leg lengthening with the callotasis method in which callus formation was poor.