Background: The aim of this study was to compare the effects of two types of cast immobilization (human position cast and dynamic cast) on hip development in children with Developmental dysplasia of the hip (DDH) after closed reduction (CR)., Methods: A retrospective study of 60 children (64 hips) with DDH who underwent CR and cast immobilization between January 2015 and December 2016 at our Institution was performed. The average age at the time of CR was 14.6 months (range, 6.1-23.5). Fifty-seven females and 3 males were included. According to the technique of cast immobilization, two groups of patients could be identified: patients with DDH managed by human position cast immobilization (group A: 32 patients, 34 hips) and patients with DDH treated by dynamic cast immobilization (group B: 28 patients, 30 hips). Hip joint distance (HJD) after CR was measured on MRI. Acetabular Index (AI) and Acetabular Depth Radio (ADR) were measured of anterior-posterior (AP) radiographs before and 3 months after CR; AI and central edge angle (CEA) were measured last follow-up AP radiographs. The presence of subluxation or dislocation and avascular necrosis (AVN) at the last follow-up visit was also evaluated., Results: The patients were comparable regarding to sex, side, age, Tönnis degree, AI, and ADR before the reduction between two groups. There was no significant difference in HJD improvement between the two groups 6 weeks following closed reduction. The AI (27.5±5.1°) of group B was significantly lower than those of group A (31±4.9°) (P=0.03) when cast was removed 3 months after CR. At the last follow-up, the incidence of AVN was similar between the two groups of patients (group A: 11.8% versus group B: 13.3%), and the incidence of subluxation or dislocation (group A: 8.8% versus group B: 10%). At last follow-up visit, the AI (23.7±5.4°) in group B was significantly lower than in group A (26.9±4.1°) (P=0.02)., Conclusions: Dynamic cast immobilization promotes acetabular development following CR in patients aged 6 to 24 months with DDH. Dynamic cast immobilization does not increase the risk of dislocation or subluxation, nor of AVN.