To analyze the effectiveness of anterior cruciate l igament (ACL) reconstruction with preserving the remnant ACL by allograft l igament. Methods Between January and July in 2008, 97 patients underwent ACL reconstruction with the allograft l igament. On the basis of the ACL’s condition, patients were divided into 2 groups. In the trial group (patients having remnant ACL, n=38), there were 27 males and 11 females with an average age of 24.3 years (range, 16-43 years); of them, 8 cases compl icated by menicus injury and 13 cases by Outbridge I degree cartilage injury; the time from injuryto operation was 3-20 weeks (mean, 8.6 weeks). In the control group (patients having no remnant ACL, n=59), there were 35 males and 24 females with an average age of 27.8 years (range, 18-48 years); of them, 16 cases compl icated by menicus injury and 23 cases by Outbridge I degree cartilage injury; the time from injury to operation was 4-44 weeks (mean, 12.7 weeks). All injuries were caused by sports in 2 groups. All patients had positive anterior drawer test and positive Lackman test. Before operation and 3, 8, 9, 15 months after operation, the function of the knee joint was evaluated by the International Knee Documentation Committee (IKDC) scale and Lysholm score. KT-2000 arthrometer was used to evaluate knee laxity. Results All incisions healed primarily, and no intraarticular infection occurred. Thirty-seven cases and 57 cases were followed up 15 months in the trial group and the control group, respectively. Knee instabil ity disappeared; the patients showed negative anterior drawer test and negative Lackman test. By the evaluation of KT-2000, the joint sl ippage of 2 groups were less than 2 mm at 3 and 6 months after operation, but it was more than 2 mm in 2 cases of the trial group and in 3 cases of the control group at 9 months after operation; and it was no change in the trial group and 2.5-4.0 mm (mean, 3.4 mm) in the control group at 15 months after operation. Both the IKDC scale and Lysholm score were higher in the trial group than in the control group 3 months after operation, but the differences were not significant (P gt; 0.05). Both the IKDC scale and Lysholm score were