PURPOSE: To describe the magnetic resonance (MR) imaging findings of non-traumatic thickening of the anterior cruciate ligament (ACL) and to evaluate the associated lesions. MATERIALS AND METHODS: Between January 2003 and August 2005, 44 knees of 44 patients who had thickened ACLs on MR images and had no history of knee trauma were analyzed retrospectively. The normal thickness of the ACL was measured on axial T2-weighted images of 40 healthy adult knees. The MR imaging findings of the thickened ACLs and associated lesions were analyzed. RESULTS: In 40 cases of healthy knees, the thickness of the proximal ACL was 3-6 mm. In 44 cases of non-traumatic thickening of the ACL, the thickness of the proximal ACL was 8-14 mm. There was an increased signal intensity and ill-defined border in all cases of thickened ACLs, linear low-signal intensity fibers parallel to the long axis of the thickened ACL (celery stalk appearance) in 24 cases, and entrapment in 10 cases. With respect to associated lesions, there was osteoarthritis in 40 cases, meniscal tears in 42 cases, and degeneration of the posterior cruciate ligament in 7 cases. CONCLUSION: Non-traumatic thickening of the ACL was associated with osteoarthritis and meniscal tears in almost all cases and showed increased signal intensity and ill-defined borders simulating acute ligamentous tears.