We investigate the serial change of the bone density of the lengthening sites in distraction osteogenesis in long bone lengthening of the lower extremity by measuring the pixel value of the PACS(Picture Archiving Communication System). The purpose of this study was to find the clinical implication of the pixel value in PACS in the distraction osteogenesis. The number of the distraction sites were 22 in tibia and 16 in femur. The average distraction length was 4.5cm ranged between 2.1cm and 7.0cm in femur, 4.1cm ranged hetween 1.9cm and 6.8cm in tibia. When the image were sent to the PACS workstations, they were directly interfaced to the workstation without any processing. The absolute and the relati ve pixel values of cortical bones of the original and the lengthening sites repr sented in workstation of PACS were obtained by average value measuring 3 times by 3 different persons. The average absoiute pixel value of the original cortical bone near distraction site was not significantly changed, maintaining 575±6 in femur, and 570±7 in tibia. The absolute pixel vaIues in AP and lateral view were not significantly changed until 6 week/cm, but rapidly increased after 7 week/cm hoth in the tibia and the femur. The relative pixel value of the lengthening sites were more than 95% in three of the four cortices at the time of the removal of the external fixators. in conclusion, the pixel value of the PACS can be a rapid, simple and easy method for detection of the change of the bone density in distraction osteogenesis.