PURPOSE: The purpose of this study was to investigate the relation between knee functionality and posterior tibial slope in anterior cruciate ligament (ACL)-deficient and ACL-reconstructed patients. METHODS: Patients with isolated ACL injuries on the surgical waiting list and patients who underwent ACL reconstruction with bone-patellar tendon-bone grafts between 18 and 24 months after surgery were recruited from the orthopaedic sports injury clinic. The study included 44 ACL-deficient patients (range 16-49) with a mean age of 26.4 years and 24 ACL-reconstructed patients with a mean age of 27.2 years (range, 25 to 49 years). Posterior tibial slope was measured on a digitalized lateral radiograph by use of the posterior tibial cortex as a reference. The Cincinnati scoring system was used to assess knee functionality. RESULTS: The posterior tibial slope averaged 6.10° ± 3.57° (range, 0° to 17°) in the ACL-deficient group and 7.20° ± 4.49° (range, 0° to 17°) in the ACL-reconstructed group. An anterior tibial slope was not measured in any of the participants. The mean Cincinnati score was 62.0 ± 14.5 (range, 36 to 84) in the ACL-deficient patients and 89.3 ± 9.5 (range, 61 to 100) in the ACL-reconstructed patients. There was a moderate but nonsignificant correlation (r = 0.47) between knee functionality and slope in the ACL-deficient patients. When we divided posterior tibial slope into intervals of 0° to 4° (mean score, 58.4), 5° to 9° (mean score, 59.6), and greater than 10° (mean score, 75.4), a strong significant correlation (r = 0.91, P = .01) was observed between knee functionality and slope. There was a weak but nonsignificant correlation (r = 0.24) between knee functionality and slope in the ACL-reconstructed patients. When we divided posterior tibial slope into intervals of 0° to 4° (mean score, 78.2), 5° to 9° (mean score, 86.1), and greater than 10° (mean score, 89.4), a strong and significant correlation (r = 0.96, P = .0001) was observed between knee functionality and slope. CONCLUSIONS: The results of this study suggest that ACL-deficient and ACL-reconstructed patients with higher posterior tibial slope have more functional knees.