We describe the case of a patient with adventitial cystic disease of the popliteal artery in which a direct anatomic communication between the cysts and the nearby knee joint was demonstrated by magnetic resonance imaging and confirmed by surgery. This unusual observation could shed some light on the much debated question of the cause, the pathogenesis, and the management of the affection. Moreover, it emphasizes the importance and the role of magnetic resonance imaging in the diagnosis of adventitial cystic disease of the popliteal artery.