An uncommon case of traumatic arteriovenous fistula of the left internal mammary artery following penetrating chest trauma is presented. The patient developed a left parasternal machinery murmur six days after thoracotomy to repair a pericardial tamponade after a penetrating left parasternal stab wound. Selective digital subtraction angiography revealed pseudoaneurysm formation with fistulous connection of the left internal mammary artery to the adjacent vein. The fistula was successfully occluded with coil embolization.