Ilizarov method has recently been worldwidely popularized in the management of complicated fracture, limb shortening and complex deformities. Determination of the quantity and quality of the callus in the distracted gap is difficult, particulary in the early stage of lengthening by the plain radiography. Ultrasonography has recently been proposed as one of the tools which enables earlier and precise evaluation of the callus formation in the distracted gap. In 14 patients with 27 distracted sites, the callus formation was evaluated by ultrasonography to control the rate and rhythm of distraction between January 1990 and May 1991 at Chungnam national University Hospital. The results are as follows; 1. There were ten male and four female patients with mean age at operation was 24.8 years ranging between 2 and 48 years. 2. Seven patients had infected nonunion, two patients with sequela of poliomyelitis, two patients with genu varum deformity, one with dwarfism, one with neurofibromatosis and one of congenital pseudoarthrosis of tibia. 3. Eleven distraction sites were in the tibia, and six was the distal femur. 4. Fourteen patients underwent sonographic examination at 1,2,3,4,5,6 week after lengthening. Detection of sonographic echogenic foci is evident in 19.6 days (range 10-36 days) after lengthening, and detection of osteoid formation on plain radiography is 63.9 days (range 43-88 days) after lengthening. New bone formation could be detected with sonography 44.3 days (range 33-52) earlier than with the plain roentgenogram. 5. Twelve patient who have good and early callus formation on USG, also have good callus formation on plain roentgenogram. 6. In 2 cases, We could find 1cm and 1.5cm sized cyst with USG at 3 weeks after lengthening. In a case, We got the normal osteogenesis within cyst after percutaneous needle aspiration. In other case, We could achieve the normal osteogenesis in the cystic lesion after cycle of compression and distraction. 7. In 10 patients who have pain on pin site, We can find the thickened and elevated periosteum and fascia with hypoechoic lesion near the pin insertion site.