Background:Recent studies have shown that lateral menisci have a higher healing potential and that they can be treated successfully without symptoms by being left in situ during anterior cruciate ligament (ACL) reconstruction. However, few studies have reported morphological results.Hypothesis:Stable posterior horn tears of the lateral meniscus left in situ during ACL reconstruction could be healed spontaneously and would result in not only successful clinical outcomes but also morphological restoration.Study Design:Cohort study; Level of evidence, 3.Methods:Among 367 patients who underwent primary ACL reconstruction between 2008 and 2010, 53 patients who had lateral meniscus tears that were left in situ were analyzed. These patients were evaluated subjectively and radiologically and compared with a matched control group that underwent ACL reconstruction without any other structural disorders. Of the 53 patients with stable posterior horn tears of the lateral meniscus left in situ, 28 patients were assessed by second-look arthroscopic surgery and magnetic resonance imaging (MRI).Results:The mean follow-up of the study group and the control group was 36.47 and 37.26 months, respectively. There were no statistical differences in postoperative clinical outcomes between the 2 groups. Clinical results of both groups including the Lysholm score, Tegner activity score, and International Knee Documentation Committee (IKDC) score significantly improved. In the subgroup composed of 28 patients, follow-up MRI showed 25 (89%) and 24 (86%) healed menisci in sagittal and coronal views, respectively. Twenty-one (75%) were considered to be completely healed, and 5 (18%) were incompletely healed on second-look arthroscopic surgery.Conclusion:Stable posterior horn tears of the lateral meniscus left in situ at the time of ACL reconstruction revealed successful clinical outcomes compared with isolated ACL injuries and showed considerable healing and functional restoration of tears with repeat MRI and second-look arthroscopic surgery. Therefore, leaving stable posterior horn tears of the lateral meniscus in situ during ACL reconstruction should be considered.