OBJECTIVE The purpose of this present study was to examine contralateral ACL injury cases after ACL reconstruction, to determine the characteristics of such injuries. METHODS We performed a retrospective analysis of 24 patients with contralateral ACL injury after ACL reconstruction. The control group consisted of 200 cases with unilateral ACL injury. The following were examined in the contralateral group: timing of the contralateral ACL injury, and the situations of the initial and contralateral ACL injuries. The following items were compared between the contralateral and control groups: age at the time of initial injury, level of competitive sports using Tegner activity scores, knee anterior laxity (KT-1000), and the ratio (%) of affected to unaffected legs in the strengths of the knee extensor and flexor muscles 6 months after surgery. RESULTS Examination of injury situations showed that approximately 70% of the contralateral group was injured in situations similar to those at their initial injuries. There were no significant differences between the two groups in age at the time of initial injury , Tegner activity scores, knee anterior laxity, and the strengths of the knee extensor, flexor muscles and H/Q ratio 6 months after reconstruction. But, the age at the time of initial injury trended to be low in contralateral group. CONCLUSIONS Knee anterior laxity and muscle weakness of the reconstructed legs six months following surgery were not individually related to contralateral ACL injury occurring approximately two years after surgery.