It is controversial to perform bone surgery at the time of open reduction in developmental dislocation of the hip in children 12-18 months old. The purpose of this study is to investigate whether concomitant bone surgery is necessary in patients treated with medial open reduction in this age range. Patients that were under 12 months of age at the time of open reduction were compared with patients that were 12-18 months old. Forty-four hips of 30 patients treated with open reduction through Ferguson's medial approach have been included in the study. Mean follow-up was 19.6 years (13-275). Age at the time of open reduction was less than 12 months in 21 hips (group A) and 12 months or more in 23 hips (group 13). There was no significant difference between two groups concerning avascular necrosis or unsatisfactory radiological outcome (Severin's groups III and IV). A higher rate of secondary bone surgery was necessary in group B than in group A. Although secondary bone surgery is needed at a higher frequency in children 12-18 months old, the radiological outcome is not significantly different for patients younger than 12 months. Therefore, the recommendation of concomitant bone surgery on a routine basis during open reduction in developmental dislocation of the hip in children 12-18 months old is debatable.