THE LOCALIZATION of block in the main branches of the bundle of His was clarified by the studies of Wilson and his co-workers.1In this regard precordial electrocardiography proved to be of great value. The clinical significance of these findings, however, has been confusing, and it is well known that right or left bundle branch block is commonly associated with coronary artery disease. Bundle branch block may also occur in valvular disease, particularly with aortic stenosis. Furthermore, some evidence has appeared to indicate that right bundle branch block in general may indicate a better prognosis than left bundle branch block.2Recently, it has been observed that right bundle branch block, complete or incomplete, is a common finding in patients with congenital cardiac disorders, especially those with atrial septal defects.3It is the main purpose of this study to call attention to a group of patients who have