Introduction: Anterior cruciate ligament (ACL) reconstruction requires a detailed analysis of the posterior tibial slope (PTS) as excessive values may cause the reconstruction to fail and require a slope-decreasing anterior closing wedge tibial osteotomy combined with revision of the failed ACL reconstruction. The main purpose of this study was to assess the accuracy of correction after slope-decreasing anterior closing wedge tibial osteotomy in cases of chronic anterior instability caused by ACL rerupture. Materials and methods: This single-center retrospective study included 19 patients (20 knees) operated on by slope-decreasing anterior closing wedge tibial osteotomy combined with a second revision ACL reconstruction. The mean age was 22.4 ± 3.3 years and the mean follow-up was 12.7 ± 4.4 months. The preoperative planning was based on lateral calibrated X-rays of the entire tibia. The height of the closing wedge, which corresponded to the base of the osteotomy, was measured in millimeters. The procedure was performed using the freehand technique. The accuracy of the correction was defined as the difference between the desired preoperative PTS and the postoperative PTS achieved. An inter- and intraobserver analysis was performed. Results: The mean preoperative PTS was 13.9 ± 2◦ and the mean postoperative PTS was 4.0 ± 1.7◦. The mean PTS correction was 10.1 ± 2.1◦ with a planned target of 5.4 ± 1.8◦. The accuracy obtained between the planned target and the postoperative corrections was 1.7 ± 1.1◦. The regression analysis showed that the accuracy of the PTS correction was not influenced by the patient’s age, BMI, excessive preoperative PTS, or degree of correction achieved (p > 0.05). Conclusion: Slope-decreasing anterior closing wedge tibial osteotomies performed using the freehand technique for ACL graft rerupture can correct an excessive PTS within 2◦ of the planned slope correction. This accuracy is not determined by demographic factors, excessive preoperative PTS or degree of correction achieved. Level of evidence: IV; retrospective cohort study.