The authors report the clinical case of a 50-year-old man who was observed in the emergency department because of a one week long, painless left lower limb swelling. At presentation, a gross edema was evident and peripheral pulses were present and symmetrical. The ultrasound scan findings suggested short occlusion in the transition of the common femoral (CFV) to the external iliac veins. A venography confirmed a filling defect in the above mentioned location, with normal findings throughout proximal and distal veins. On CT scans a cyst measuring 20mm of maximal diameter was disclosed, medial to the common femoral artery, compressing the CFV and seemingly continuous to the hip joint. He underwent surgical correction through open inguinal approach that allowed to confirm a complete compression of the CFV by the cyst against the inguinal ligament. The CFV recovered its normal aspect after cyst, removal and the patient was discarded home at day 2 after surgery, with complete regression of the oedema wich is maintained one year after the event, as well as absence of local ultrasound abnormalities. The hip joint is a rare location for synovial cysts. Synovial or adventitial cysts in that area may compress de CFV. The histological tests are useless for distinguishing between the two diagnosis. The clinical presentation consists of an inguinal mass or lower limb oedema due to CFV compression. Given its location underneath the vein, these masses are often difficult to detect. Vein decompression is suggested to avoid deep vein thrombosis. When a synovial cyst is present it is possible to find a dissecting plane between the two anatomical structures. Though feasible, percutaneous cyst aspiration is linked to high recurrence rates.