Purpose: This retrospective study was designed to determine the type and frequency of associated lesions in patients with chronic lateral ankle instability who had modified Brostrom lateral ankle ligament reconstruction. Materials and Methods: Between 2004 and 2007, 60 cases of 60 patients were enrolled in this study. A retrospective review of the magnetic resonance images of the affected ankle was conducted by two orthopedic surgeons who did not get any information about intraoperative findings and the lesions were admitted when two doctors were coincident. Results: The overall incidence of associated lesions found in this study was about 83%. Peroneal tenosynovitis was the highest frequency (32%), followed by osteochondral lesion of talus (28%), anterolateral impingement (15%), Os subfibula (13%), Os trigonum (12%), ankle synovitis (12%), anterior tibiofibular ligament tear (15%), anterior bony spur (7%). Another findings were loose bodies (5%), flexor tendon tenosynovitis (5%), medial osteophyte (3%). Conclusion: Identifying these associated lesions will be helpful in treating chronic lateral ankle instability especially when the surgeon have a plan to operate the instability. We suggest that the better results can be obtained when the associated lesions are corrected simultaneously.