In order to evaluate the factors causing complications and affecting the results, we reviewed 12 cases in which the deformities of upper extremities were corrected by the Ilizarov method at Seoul National University Hospital from October 1989 to May 1991. The average age of patients was 10 years and 6 months(range, 4 years and 7 months-15 years and 7 months). The causes of length discrepancies and deformities were as follows ; post-traumatic physeal injuries 3, post-infectious physeal injuries 3, osteochondromas 2, congenital ulnar club hand 1, congenital radio-ulnar synostosis 1, brachial plexus injury 1, residual poliomyelitis l. The goals of treatment were bone lengthening in 6 cases, bone lengthening and deformity correction in 4, bone lengthening and osteosynthesis in 1, and arthrodesis in 1. The results were summarized as follows; 1. Initial length discrepancy of upper extremity averaged 5.1cm(range, 1.5-8.3cm). 2. In humerus, average bone lengthening was 5.3cm(range, 4.0-7.0cm), percentage increase and healing index averaged 27% and l.lmos/cm, respectively. 3. In radius, average bone lengthening was 2.8cm (range, 0.9-3.7cm), percentage increase and healing index averaged 18% and 2.5mos/cm, respectively. 4. In ulna, average bone lengthening was 3.3cm (range, 1.8-6.0cm), percentage increase and healing index averaged 24% and 2.0month/cm, respectively. 5. In all of the cases, we could achieve the goals of lengthening. In case of shoulder arthrodesis, bony union was achieved at 4 months. 6. There were 7 cases of problems (58%), which were delayed consolidations 4, transient ulnar nerve palsy 1, limited range of motion of elbow 1, and pin tract inflammation 1, and there was 1 case of obstacle (8%), which was ectopic ossification at the pin insertion site. In conclusion, the Ilizarov method was useful for not only bone lengthening and deformity correction, but also arthrodesis with the advantage of postoperative readjustment of the segment. Precise application of Ilizarov method and proper post-operative management affect the results and are essential for prevention of complications.