The lengthening of the extremities and correction of deformitles by using Ilizarov method is rather a recent treatment method in Korea. After corticotomy, lengthening of extremity is performed at an average speed of 1 mm per day. The most important factor for the success of limb lengthening is the good osteogenesis in the distraction gap. Early diagnosis of good osteogenesis is, therefore very important for the good result. With the conventional radiography, the diagnosis of early osteogenesis is possible at least 4 to 6 weeks after distraction. But it is possoble at 2 weeks after distraction with the ultrasonography. Ultrasonography has a definite advantage over conventional radiograph for the early diagnosis of osteogenesis. Authors reviewed 28 cases of osteogenesis in the distraction gap during Ilizarov lengthening with the ultrasonography from August 1990 to June 1991 in Sang Gye Paik Hospital. The results were as follows; 1. In 25 Cases, we could observe osteogenesis in the distraction gap during lengthening at 2 and 4 wddks by using the ultrasound. We could confirm the good osteogenesis in 25 cases with the conventional radiograph in the follow-up study. 2. In 2 cases, we could observe cyst formation in the distration gap with ultrasonography. we could achieve osteogenesis in the cystic region after cycles of compression and distraction. 3. In 1 case, there were delayed ossification although the ultrasound showed normal osteogenesis. We could not get normal osteogenesis in the follow-up study. Accoding to the results above, we can conclude that ultrasound is an advisable method for the early diagnosis of osteogenesis in the distraction gap during lengthening.