Developmental dysplasia of the hip (DDH) is a common pediatric malformation and one of major causes of juvenile disability. Quantitative imaging plays an important role in early diagnosis of DDH[1]. Although MRI has been widely applied to evaluate pediatric patients with DDH with a variety of parameters [2,3], the in-depth study only for acetabulum morphology is still required, especially the correlation of different pathological changes of acetabulum[4]. This study aimed to investigate the efficacy of MRI for evaluating morphology and long-term prognosis of acetabulum in pediatric patients with DDH.47 subjects (mean age=17.42±10.19 months, 106 hips) with DDH underwent MRI examination on a 3T Scanner (Achieva Intera; Philips Medical Systems, Best, the Netherlands) with following parameters: PDw: TE / TR=3800/30ms; FOV/ thickness= 260 *260/2mm; matrix=256 *128 ;T2wTE / TR=5961/120ms; FOV/ thickness= 280 *260/2mm; matrix=256 *128 ), The MRI of subjects with DDH after the first concentric reduction were analyzed retrospectively and of all infants with DDH 35 intact hips were classified in control group and the other 59 hips were classified in DDH group. According to whether the concentric relationship was still kept between the femoral head and acetabulum at least one year later when the first concenric reduction achieved, the DDH group was further divided into two subgroups: non-reduced group (37 hips ) and reduced group (22 hips ). Then the bony acetabular index (BAI), cartilaginous acetabular index (CAI), acetabular anteversion index of bone (BAAV) and cartilage (CAAV) were measured and cartilaginous index (CI=(BAI-CAI) / BAI) was computed. The differences and relationships of the parameters among these groups were compared and analyzed respectively. All analyses were performed using SPSS 18.0 Statistical Software (SPSS Inc., Chicago, IL, USA). P-values < 0.05 were considered statistically significant.Significant differences were found in the BAI (t=8.273, P <0.001), CAI (t=5.255, P <0.001), BAAV (t=6.157, P<0.001) and CAAV (t=4.489, P<0.001) between the DDH group and control group (Table1), however, no obvious differences between non-reduced group and reduced group were found. Moreover, there was a positive correlation among the BAI and CAI in the DDH goup (r = 0.493, P <0.01) and control group (r = 0.554, P <0.01). There was obvious differences with statistical significance in the CI between non-reduced group and reduced group (t=-2.315, P=0.24) (Figure1), no statistical differences were found in CI between DDH group and control group, as well as between Tonnis subgroups of the DDH goup (Table2), and age was negatively correlated with the CI (r = -0.345, P =0.01) (Figure2) . Compared to other imaging modality, MRI can non-invasively provided information of the osseous and cartilaginous acetabular anatomy without any radiation exposure[5]. In our study MRI can effectively evaluate the alteration of children ‘s acetabulum morphology. The bony changes of acetabular index in patients with DDH were mainly located in the superior-exterior and posterior edges of the acetabulum, and cartilaginous changes in the superior-exterior and anterior edges of the acetabulum(Figure3). Previous studies mentioned the changes of direction of acetabulum were also found [6] . Our study further discovered that there was a significant difference in the value of CI between non-reduced group and reduced group. Therefore MRI could potentially be used as a sensitive and effective tool for the diagnosis of DDH in an early stage by evaluating the acetabulum morphology.MRI can effectively evaluate the alteration of children’s acetabulum morphology. The CI can preliminarily predict the long-term prognosis of DDH after reduction.