Background: Patients with developmental dysplasia of the hip (DDH) are at risk for residual acetabular dysplasia even after successful closed reduction. The aim of this study was to identify predictors of long-term outcomes in order to guide prognostication and management., Methods: Patients who were treated for DDH at 2 institutions between 1970 and 2010 and had follow-up until skeletal maturity were screened for inclusion. Hips that underwent open reduction were excluded to reduce iatrogenic confounding. Syndromal (including neuromuscular and arthrogrypotic) hip instability with collagenopathies were excluded. Demographic characteristics, Tönnis grade, age at the time of the reduction, surgical treatment, acetabular index, lateral center-edge angle, residual dysplasia graded using the Severin classification, and the presence and type of proximal femoral growth disturbance categorized using the Bucholz and Ogden classification were assessed. In addition, the the acetabular angle was recorded at the latest follow-up before secondary procedures, and the Oxford Hip Score and 5-level EuroQoL (EQ)-5 Dimension score were recorded at the latest follow-up. To account for repeated measures, generalized estimating equations (GEE) logistic regression was utilized for the multivariable analysis. A support vector machine model and a receiver operating characteristic curve analysis were utilized to identify prognostication thresholds., Results: One hundred and seven hips (96 female, 11 male) that were followed to skeletal maturity, with a mean follow-up of 20 years (range, 10 to 54 years), were included in the analysis. Eighty-nine hips (83%) demonstrated a good outcome at skeletal maturity, with a Severin grade of I or II. Major growth disturbances of Bucholz and Ogden types II, III, or IV were present in 13 hips (12%). At the latest follow-up after skeletal maturity (before any secondary procedures), the mean acetabular angle was 45° ± 4° and the mean lateral center-edge angle was 26° ± 8°. The mean Oxford Hip Score and EQ visual analog scale values were 47 and 86, respectively. A GEE logistic regression analysis of 1,135 observations revealed that the acetabular index (odds ratio [OR], 1.16 per degree; p < 0.001) and age (OR, 1.20 per year; p = 0.003) were significant predictors of a poor outcome (i.e., Severin grade III, IV, or V). Significant differences in acetabular indices across all age groups were found between hips with a good outcome and those with a poor outcome. Age-specific acetabular index prognostication cutoff values are presented., Conclusions: This long-term follow-up study demonstrated that the age-specific acetabular index remains an important predictor of residual dysplasia at skeletal maturity. The proposed prognostication chart and thresholds herein can help to guide orthopaedic surgeons and parents when contemplating the use of an intervention versus surveillance to optimize long-term outcomes., Level of Evidence: Prognostic Level III. See Instructions for Authors for a complete description of levels of evidence., Competing Interests: Disclosure: This work was funded by the Sanming Project of Medicine in Shenzhen. The study sponsors had no role in the study design, data collection, analysis, interpretation, writing of the report, or submission decision. The Disclosure of Potential Conflicts of Interest forms are provided with the online version of the article (http://links.lww.com/JBJS/I194)., (Copyright © 2024 The Authors. Published by The Journal of Bone and Joint Surgery, Incorporated.)