Cystic degeneration of the adventitia of the popliteal artery is a rare clinico-angiographic diagnosis in the chronic ischaemia syndrome of the lower extremity. If the three cardinal signs occur concomitantly, namely, intermittent ischaemia syndrome, unilocular manifestation near the joint and clearly limited filling defect in the arteriogram, diagnosis can be made with a high measure of certainty. Among our angiographically examined patients, we observed, in the course of one year, three male patients who presented the full picture of the disease with regard to clinic, localisation and angiographic findings. Intraoperatively it was not possible to establish a connection between adventitia cyst and neighbouring articular capsule; this applies, in fact, to all our cases. Histologically we found in one case, in the hollow spaces of the adventitia, an indication of the possible existence of a synovial recess, so that the scattering of mucine-secreting endothelium of the joint into the vascular adventitia is again brought up for discussion.