Background: The posterior cruciate ligament (PCL) index quantifies the curvature of the PCL seen on magnetic resonance imaging (MRI) that is caused by a change in tibiofemoral alignment in the anterior cruciate ligament (ACL)-deficient knee. It has been proposed that the PCL index may be useful in assessing the success of ACL reconstruction (ACLR). Hypothesis/Purpose: The purpose of this study was to test if the PCL index is predictive of in vivo rotational kinematics and joint laxity measures in knees of patients after single-bundle ACLR. The hypothesis was that the PCL index is predictive of rotational knee kinematics and objective laxity scores. Study Design: Controlled laboratory study. Methods: At a mean of 18.1 months postoperatively, the PCL index was defined on MRI in 16 patients prospectively followed up after anatomic single-bundle ACLR and in 16 matched controls. The patients were evaluated with 3-dimensional motion analysis during (1) descending and pivoting as well as (2) landing and pivoting tasks. The side-to-side difference in tibial rotation range of motion between the reconstructed knee and the contralateral intact knee was calculated. The side-to-side difference in anterior tibial translation was measured with a KT-1000 arthrometer. Linear regression models were used with the PCL index as a predictor of the side-to-side difference in tibial rotation for each task and the side-to-side difference in anterior tibial translation. Results: The PCL index of the reconstructed knees was significantly lower compared with that of the control knees (P< .001). The index was predictive of the side-to-side difference in tibial rotation during both tasks (R 2 = 0.472 and 0.477, P = .003), with a lower index being indicative of increased rotational laxity. It was not predictive of anterior tibial translation (at 134 N: R2 = 0.13, P = .17; at maximum force: R 2 = 0.009, P = .726). Conclusion: The PCL index after anatomic single-bundle ACLR using a bone-patellar tendon-bone graft is predictive of rotational kinematics during in vivo dynamic pivoting activities. The results show that the PCL index is correlated with the postoperative ability to control rotational kinematics of the knee joint. Clinical Relevance: This study provides evidence regarding the interplay between restoration of the native ACL's anatomy and the PCL's appearance and suggests that the effective restoration of tibiofemoral alignment after ACLR that is reflected in the PCL index translates into better functional outcomes as measured by tibial rotation during pivoting activities. �� 2013 The Author(s).