Purpose To evaluate the correlation between avascular necrosis (AVN) and the amount (severity) and direction (translation and angulation) of initial displacement of pediatric femoral neck fractures. Methods We retrospectively reviewed 108 pediatric patients (mean age 10.3 +/- 4.1 years) with femoral neck fractures. The amount of initial translation (T) and angulation (A) was measured on anteroposterior (AP; T-AP% and A(AP)) and lateral (T-L% and A(L)) radiographs. The direction of translation was determined on AP (medial or lateral) and lateral radiographs (anterior or posterior). Furthermore, the presence of a comminuted medial cortex on the AP pelvis radiograph was also recorded. Logistic regression analysis, receiver operating characteristic (ROC) curve analysis, student's t tests, and chi-square tests were used to evaluate the correlation between AVN and the severity and direction of displacement. Results Twenty-eight out of 108 hips (25.9%) developed AVN of the femoral head. Logistical regression analysis indicated that T-AP%, T-L%, A(AP), and A(L) were risk factors for AVN (P < 0.05). The analysis of ROC curves found that T-AP% over 37.4% and T-L% over 29% were the cut-off values for an increased incidence of AVN; similarly, A(AP) over 8 degrees and A(L) over 18.6 degrees were the cut-off values for an increased incidence of AVN. The amount of initial translation is a better predictor of AVN than angulation is; fractures with posterior translation (P = 0.002) and/or medial comminution had a significantly higher incidence of AVN (P = 0.005). The mean diagnostic accuracy of translation (74-75%) was significantly higher than that of angulation (65-66%). Conclusions Displacement severity and direction are important radiological parameters to be assessed in children with femoral neck fractures. Initial translation better predicts AVN than angulation does. Posterior translation and medial comminution are associated with an increased risk of AVN.