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Localized anterior fibrosis (cyclops lesion) is a known cause of extension loss of the knee after anterior cruciate ligament (ACL) reconstruction. We describe MR imaging as a noninvasive diagnostic tool to examine cyclops lesions.Thirty-three MR studies of 31 patients with residual persistent extension loss after ACL reconstruction using patellar tendon autograft were reviewed and compared with results of second arthroscopy. We used MR imaging to describe the ACL graft signal intensity and course, tibial and femoral tunnel placement. quantitative measurements of notch size and shape, and the presence or absence of cyclops lesions. When a cyclops lesion was revealed on MR imaging, the signal-intensity characteristics, location, and size were documented. Preoperative MR imaging findings were then correlated with findings at arthroscopy.The sensitivity, specificity, and accuracy of revealing a cyclops lesion on MR imaging were 85.0%, 84.6%, and 84.8%, respectively. We found no statistically significant differences in the size of intercondylar notches for patients with and patients without cyclops lesions.MR imaging was sensitive, specific, and accurate in revealing cyclops lesions in a subgroup of patients with extension loss after ACL reconstruction.