A case of anomalous (subaortic) position of the left brachiocephalic vein was incidentally detected on computed tomography images. Magnetic resonance angiography was performed to demonstrate the relationship of this vessel with other vascular structures. The anomalous vein was formed by the union of the left internal jugular and left subclavian veins. This vein passed downward along the left lateral side of the aortic arch, entered the aorticopulmonary window, descended in the mediastinum between the ascending aorta and the trachea and joined with the right brachiocephalic vein to form the superior vena cava. No cardiac anomalies accompanied the subaortic left brachiocephalic vein in the present case. We present the computed tomography and magnetic resonance angiography findings of this rare anomalous vein.