Positioning is critical during the placement of superior vena cava (SVC) stents in patients with malignant SVC syndrome. Although SVC stents effectively relieve various symptoms of SVC syndrome, improper stent positioning may cause life-threatening complications such as migration that result in fatal cardiac failure. Here we describe a patient with an allergy to iodinated contrast material (ICM) who presented with SVC syndrome owing to mediastinal lymph node metastases from hepatocellular carcinoma, which was successfully treated with an SVC stent. Secure stent placement was achieved by bridging the stent through the SVC to the inferior vena cava with venography using carbon dioxide instead of ICM.