Subadventitial cyst of the popliteal artery is a well known entity. We report a new case and the diagnostic and therapeutic advantages of noninvasive explorations. Transcutaneous duplex-scan provided diagnosis and showed cystic formations in contact with a segmentary occlusion of the popliteal artery. Magnetic resonance imaging (MRI) evidenced communication between the cysts and the articular capsule of the knee joint. Surgical exeresis confirmed the communication with the synovial fluid. This observation demonstrated: i) the contribution of duplex-scan for the diagnosis of adventitial cysts and arterial consequences; ii) the contribution of MRI in identifying communication with the synovial fluid; iii) the presence of arterial wall lesions (necrosis of the media) would favor no conservative treatment.