We described three pediatric cases during which we encountered a rarely described component of the atrial septum, a membranous remnant of the left venous valve of the IVC, in patients who were brought to the catheterization lab for device closure of an atrial septal defect. In all cases, this remnant was not noted by previous transthoracic imaging. Awareness of this potential anatomic variation is important to successful device closure of atrial septal defects.