“May-Thurner syndrome” (MTS) or “Cockett’s syndrome” is a vascular disease in which the left common iliac vein is compressed against the spine by the right common iliac artery, very rarely by the left or by both common iliac arteries. The congenital form is even more rare and is due to an anomaly of embryological development. This case describes compression and thrombosis of the left common iliac vein. A 12-year-old girl reported for observation with fever and arthralgia in the left lower limb. Sonography and computed tomography revealed thrombosis of the left common iliac vein caused by a congenital reduction of the iliac-vertebral space. Due to the rarity and non-specificity of the symptoms related to MTS, patients can suffer from repeated thrombosis and potentially pulmonary embolism.