Background Revision anterior cruciate ligament (ACL) surgery can be expected to become more common as the number of primary reconstruction keeps increasing.This study aims to investigate the factors causing instability after primary ACL reconstruction,which may provide an essential scientific base to prevent surgical failure.Methods One hundred and ten revision ACL surgeries were performed at our institute between November 2001 and July 2012.There were 74 men and 36 women,and the mean age at the time of revision was 27.6 years (range 16-56 years).The factors leading to instability after primary ACL reconstruction were retrospectively reviewed.Results Fifty-one knees failed because of bone tunnel malposition,with too anterior femoral tunnels (20 knees),posterior wall blowout (1 knee),vertical femoral tunnels (7 knees),too posterior tibial tunnels (12 knees),and too anterior tibial tunnels (10 knees).There was another knee performed with open surgery,where the femoral tunnel was drilled through the medial condyle and the tibial tunnel was too anterior.Five knees were found with malposition of the fixation.One knee with allograft was suspected of rejection and a second surgery had been made to take out the graft.Three knees met recurrent instability after postoperative infection.The other factors included traumatic (48 knees) and unidentified (12 knees).Conclusion Technical errors were the main factors leading to instability after primary ACL reconstructions,while attention should also be paid to the risk factors of re-injury and failure of graft incorporation.