The Ilizarov distraction osteogenesis, when applied to various traumatic nonunion, bone defects and limb deformity from congenital or acquired causes and sequelae of various bone diseases, resulted in successful limb lengthening and straightening. Though successful increase in amount of limb lengthening and straightening has been obtained, the limitation of plain roentgenographic evaluation to detect a small amount of new bone formation in one first few weeks naturally needed another study modalities like bone ultrasonography which in one detect early osteoid formation at the distraction site (Young et al 1990). Fourteen patients with twenty five distraction sites were evaluated between December 1990 and April 1991 at Yong Dong Severance Hospital, Yonsei university. The results of this study were as follows;1. There were eight male and six female patients with mean age at operation was 25.5 years ranging between 6 and 47 years. 2. Seven patients had traumatic nonunion with infection or bone loss and malunion, two patients with sequela of poliomyelitis, two with dwarfism, one with rickets, one with Ollier's disease and one with congenital shortening respectively. 3. Fourteen patients had undergone Ilizarov limb lengthening at twenty five sites, seven in femur and eighteen in tibia. 4. Twelve patients underwent weekly sonographic study until osteoid formation was evident, and two children had sonographic study to detect a complication in limb lengthening. 5. Detection of the osteoid at the distraction site was evident in 18 days (range 7-30) after lengthening, and evident trabeculation in plain roentgenogram was detected in average 38 days (range 21-64) after longthening. New bone formation could be detected with sonogram 20 days (range 7-56) earlier than with plain roentgenogram. 6. There were three patients who showed abnormal findings in sonogram:one had hematoma related to acute distraction from premature consolidation and two had collection of hematoma during distraction. They were treated successfuly by aspiration of hematoma and compression backward.