Myocardial infarction (MI) of the posterior wall of the left ventricle involves occlusion of either the left circumflex or the right coronary artery. Posterior wall MI most often occurs along with acute inferior or lateral MI; isolated posterior wall MI, however, does occur. Electrocardiographic abnormalities suggestive of acute posterior wall MI include the following (in leads V1, V2, or V3): (1) horizontal ST segment depression; (2) a tall, upright T wave; (3) a tall, wide R wave; and (4) and R/S wave ratio greater than 1.0 (in lead V2 only). Further, the combination of horizontal ST segment depression with an upright T wave increased the diagnostic accuracy of these two separate electrocardiographic findings. The additional-lead electrocardiogram using left posterior thorax leads is potentially helpful; ST segment elevation greater than 1 mm in this distribution suggests an acute posterior wall MI>