We report 2 cases of a popliteal mass of very unusual origin that induced compression neuropathy. The signs and symptoms could have been mistaken for those of a common Baker's cyst. Several recent studies have shown that the cause of Baker's cyst formation should be sought within the joint because of a communication between the gastrocnemio-semimembranosus bursa and the joint cavity. These 2 cysts had no communication with the articular joint, thus suggesting that the surgeon perform an open exploration of the popliteal fossa in the search for other cystic formations with origins and features different from Baker's cysts.