This study evaluates improvement of the acetabular index (AI) in patients with developmental dysplasia of the hip at 4 years after closed reduction, and determines the association between the final AI and a set of factors. Sixty-two patients (74 hips) treated with closed reduction were categorized into three groups according to age: group A (0–12 months, 18 hips), group B (13–18 months, 24 hips), and group C (>18 months, 32 hips). There was no difference in AI among the three groups before reduction (P=0.293). In groups A and C, the AI decreased significantly over time until 3 years after reduction and no differences were observed between the time points of 3 and 4 years. At 4 years after reduction, the AI of group C was significantly higher than that of groups A (P<0.001) and B (P=0.012). The overall AI improvement rate was 28.63%. The AI improvement rate of group A was significantly higher than that of group C (P=0.005). Pearson correlation analysis indicated no correlation between center–head distance discrepancy and the final AI (P=0.811). Linear regression suggested that age and initial AI correlated significantly with the final AI (R2=0.617, F=15.031, P<0.001). Other factors, such as sex, center–edge angle of Wiberg, bilaterally involved, and avascular necrosis of the femoral head, showed no correlations with the final AI (P>0.05). According to the coefficients, initial AI (β1=0.432, P<0.001) had greater effect than age (β2=0.197, P=0.023) on the final AI. In conclusion, the AI decreases in all patients after reduction and stabilizes at 3 years after reduction. The AI improvement rate is correlated negatively with age. Age and initial AI are early predictors of the progress of AI after closed reduction in developmental dysplasia of the hip patients.