Loss of full extension after anterior cruciate ligament (ACL) reconstruction, with development of an audible and palpable "clunk" with terminal extension was first described by Jackson and Schaefer as "cyclops syndrome." This syndrome, which is the result of a fibrous nodule (termed a cyclops nodule), has recently been described in patients who have sustained ACL injury but have not undergone reconstructive surgery. From 2001 to 2006, the authors identified 10 patients (five women and five men, ages 27-76 years) with cyclops nodules seen at magnetic resonance (MR) imaging. All patients had a history of trauma but no history of ACL reconstruction. The cyclops lesions had a mean size of 16 x 12 x 11 mm, with 90% of them located just anterior to the distal ACL. MR imaging showed a well-defined, somewhat heterogeneous soft-tissue nodule with a signal intensity typically similar to that of skeletal muscle. The authors suspect that the cause of cyclops lesions that occur in the absence of ACL reconstruction is similar to that suggested in the classic postoperative patient. Cyclops syndrome should be suspected in any patient in whom an ACL nodule is identified at MR imaging, and similarly a cyclops nodule should be considered as a possible cause of loss of extension in any patient who has sustained ACL injury. Movies available at http://radiographics.rsnajnls.org/cgi/content/full/e26/DC1.