The authors reviewed 50 patients of congenital dislocation of the hip who had surgical treatment at the Department of Orthopedic Surgery, Seoul National University Children's Hospital from October, 1985 to August, 1993. We investigated the contributing factors to coxa magna after surgical treatment and it's relationship to the avascular necrosis of femoral head, and observed the influnce of coxa magna on the development of the hip joint. Coxa magna was defined as a femoral head with the greatest diameter 15% greater than the opposite side. To assess the development of the acetabulum in the hips with coxa magna, radiographical horizontal and vertical parameters were measured preoperatively, and at postoperative 1 year, 2 year, and 3 year or more. Twenty-one of fifty hips had coxa manga. The incidence of coxa magna increased in patients who had open reduction at younger age(average 30 months), compared to older age(average 44 months). Neither femoral osteotomy nor pelvic osteotomy increased statistically the incidence of coxa magna. Avascular necrosis of femoral head did not necessarily resulted in coxa magna. There were not statistically significant differences in the coverage ratios of femoral head by the acetabulum between coxa magna positive and negative groups, because lateral acetabular growth accompanied in accordance with enlargement of femoral head in the majority of cases with coxa magna. Open reduction alone caused concomitant vertical overgrowth of ipsilateral hemi-pelvis with resultant pelvic tilt and leg length inequality(IHD>5mm, upto 2cm) in 6 of 10 cases. The patients, who had coxa magna with good radiological results by Severin's classification, underwent open reduction at younger age and had lesser enlargement of the femoral head, compared to those with fair or poor results.