Electrocardiography in a 77-year-old woman showed small R waves in leads V1-V3 3 hours after the onset of acute anteroseptal myocardial infarction. Abnormal Q waves appeared in leads V1-V3 only during intermittent right bundle branch block. The normal septal force disappeared after transmural septal infarction and a small force of right ventricle origin became apparent as a small R wave in V1. Right bundle branch block delayed activation of right ventricle, and thereby deleted the initial R wave and unmasked the Q wave of the septal infarction. Appearance of a Q wave in leads V1-V3 with right bundle branch block should not be assumed to reflect the extension of myocardial infarction.