Superior vena cava syndrome (SVCS) is usually caused by a malignancy or the presence of an intravascular device in a central vein. A 74-year-old male with a history of a superior vena cava (SVC) stent underwent embolisation of a brain arterio-venous malformation through the right meningeal artery with liquid Onyx. Two weeks later he presented with acute respiratory failure, upper airway obstruction, plethora, varices of the chest wall and stridor. He was intubated and placed on mechanical ventilatory support. Chest imaging revealed a linear structure in the SVC, extending to the right atrium. Interventional radiology removed the material, which was determined to be liquid Onyx. Venous pressures of the right internal jugular vein decreased after removal of the material. The symptoms resolved and patient was successfully extubated. This is the first reported case of SVCS caused by liquid Onyx.