Aim: The aim of this study was to determine the frequency of Coxa Magna (CM) which is seen during the follow-up of unilateral Developmental Dysplasia of Hip (DDH) treated with closed reduction (CR) or open reduction (AR) and to investigate the factors affecting its. Methods: Patients were divided into two groups retrospectively. Group-1: Consisted of closed reduction (CR) and pelvipedal cast under general anesthesia. Group-2: It consisted of patients who underwent medial open reduction (AR) and pelvipedal cast. At the last follow-up, femoral head diameters were measured. The diagnosis of CM was decided with 15% over-size criteria compared to the healthy side. Avascular necrosis (AVN) diagnosis Salter et al. criteria. AVN classification was made according to Kalamchi-Mc Ewen criteria. Results: There was a significant difference between the groups in terms of preoperative Tönnis stage and treatment age p = 0,000 and p = 0.003, respectively), but there was no difference in other parameters (p> 0.05). The incidence of CM was significantly higher in Group 2 (p = 0.017), but there was no difference between the groups in terms of AVN (p = 0.532). Conclusion: The main outcome of this study showed that the incidence of CM was higher in patients who underwent AR due to DDH. CM may be a non-pathologic condition associated with iliopsoas tenotomy and / or treatment. However, these CM cases may also be a result of Type 1 AVN. There is a need for more welldesigned studies with more number of cases on this subject, minimizing the factors that may affect the results.