The purpose of this work was to define the MR imaging findings of fluid collections confined to the medial collateral ligament (MCL) bursa and to correlate these findings with anatomic features shown in cadaveric specimens. The anatomic location of the MCL bursa was investigated by MR-anatomic correlation in seven cadaveric knees. The MR imaging studies and clinical charts of six patients with fluid collections confined to the MCL bursa were reviewed. On anatomic sections, the MCL bursa was located between the superficial and deep portions of the MCL. Separate femoral and tibial compartments were seen in most specimens. The anatomy of the MCL bursa is shown with MR imaging in cadaveric specimens and patients. Understanding the compartmentlike distribution of fluid in the MCL bursa at MR imaging allows accurate diagnosis and differentiation from other conditions.