Purpose To determine the diagnostic performance of MRI for helping to predict posterolateral knee instability in patients with acute anterior cruciate ligament (ACL) tear. Materials and Methods This retrospective cohort study was performed in a consecutive series of 162 patients (mean age, 32.8 years ± 10.0 [standard deviation]; 95 men [mean age, 31.0 years ± 9.6] and 67 women [mean age, 35.4 years ± 10.0]) who underwent ACL reconstruction with (n = 19) or without (n = 143) concomitant posterolateral corner (PLC) reconstruction between June 2014 and February 2017. MR images were evaluated by two radiologists. Diagnostic performance of imaging findings was calculated. Clinical evidence of posterolateral instability requiring PLC reconstruction served as reference standard. The most significant predictors of posterolateral instability were determined with decision tree analysis. Results In patients with and without PLC reconstruction, respectively, the lateral collateral ligament was completely torn in 10 of 19 (52.6%) and seven of 143 (4.9%) patients; the posterior cruciate ligament in two of 19 (10.5%) and five of 143 (3.5%) patients; the popliteus tendon in three of 19 (15.8%) and none of 143 (0%) patients; and the biceps femoris tendon in four of 19 (21.1%) and none of 143 (0%) patients (data for reader 1). The smaller structures of the PLC were not constantly viewable. Complete tear or avulsion of the lateral collateral ligament was more frequent in patients who needed PLC reconstruction (P < .001), and decision tree analysis revealed that this finding was the most statistically significant predictor of posterolateral instability. Instability was correctly predicted in 147 of 162 patients (90.7%) by reader 1 and 151 of 162 patients (93.2%) by reader 2. Conclusion Complete tear or avulsion of the lateral collateral ligament was the most significant predictor at MRI of posterolateral instability. Assessment of the smaller posterolateral corner structures did not improve diagnostic performance.