In patients with inferior wall acute myocardial infarction (AMI), the site of the culprit lesion is an important determinant of outcome. Patients with right ventricular infarction have a poor prognosis, whereas those with occlusion of the left circumflex coronary artery (LCx) have a good prognosis. Therefore, we assessed whether standard 12-lead electrocardiograms obtained on admission could identify the site of coronary artery occlusion, (i.e., a site proximal to the origin of the right ventricular branch of the right coronary artery [RCA], a site distal to the origin of the right ventricular branch of the RCA, or a site in the LCx). The ratio of ST depression in lead V3 to ST elevation in lead III (V3/III ratio) was evaluated immediately before coronary angiography in 152 patients with a first inferior wall AMI confirmed by coronary angiography within 12 hours after the onset of symptoms. For occlusion of the proximal RCA, distal RCA, and LCx, V3/III ratio was 0.2+/-0.3, 0.8+/-0.5, and 2.5+/-2.5 (p = 0.0001), respectively. The V3/III ratio <0.5 identified proximal RCA occlusion, 0.5 <V3/III ratio < or = 1.2 identified distal RCA occlusion, and 1.2 <V3/III ratio identified LCx occlusion with sensitivities of 91%, 84%, and 84%, and specificities of 91%, 93%, and 95%, respectively. We conclude that the V3/III ratio is useful in predicting the site of coronary artery occlusion in patients with inferior wall AMI.