Deveolopmental coax vara represents coax vara not present at birth but rather developing in early childhood, showing a progressive deterioration in the proximal femoral neck-shaft angle during growth. In order to determine the factors that could affect the results of corrective osteotomy, we evaluated the results of 15 developmental coax vara in 12 patients who had been treated with the femoral osteotomy at the Department of Pediatric Orthopedic Surgery, Seoul National University Children's Hospital, from February 1983 to March 1993. Of the 12 patients, there were 6 boys and 6 girls. Three patients had bilateral operations, 5 patients on the right, and the remaining 4 patients on the left. Average at the onset of symptoms was 4 years plus 5 months(range; from 1 year to 7 years plus 2 months), and average age at the tome of operation was 6 years plus 3 months(range; from 2 years plus 4 months to 10 years). We could obtain the following results: 1. Average post-operative loss of neck-shaft angle was 5% in the cases in which post-operative neck-shaft angle was converted more than 130 degrees, and was 8% in the cases in which post-operative neck-shaft angle was converted less than 130 degrees. 2. Loss of neck-shaft angle was higher during the first post-operative period, and was higher in cases in which the triangular osseous defect was persistent post-operatively. There was no correlation between the post-operative neck-shaft angle and disappearance of triangular osseous defect. 3. The femoral anteversion was converted 8.7 degrees to 27.2 postoperatively. 4. The premature arrest of the capital femoral physis was higher in cases in which the neck-shaft angle was less than 130 degrees postoperatively. 5. Leg length discrepancy, which was 2.1 cm preoperatively, did not change in cases in which the neck-shaft angle was more than 130 degrees postoperatively. However, it was converted to 3.3 cm in cases in which the neck-shaft angle was less than 130 postoperatively. 6. we could observe the femoral head deformity in 6 out of 7 cases in which the operation was performed after 7 years of age. We could draw the following conclusion based on our results: 1. We must correct the neck-shaft angle more than 130 degrees. 2. We could not equalize the leg length discrepancy by corrective osteotomy alone. 3. It may be reasonable to perform the corrective osteotomy before 7 years of age.