OBJECTIVES: This study was done to determine whether electrocardiographic (ECG) isolated ST-segment elevation (ST ↑) in posterior chest leads can establish the diagnosis of acute posterior infarction in patients with ischemic chest pain and to describe the clinical and echocardiographic characteristics of these patients. BACKGROUND: The absence of ST ↑ on the standard 12-lead ECG in many patients with acute posterior infarction hampers the early diagnosis of these infarcts and thus may result in inadequate triage and treatment. Although 40% of all acute myocardial infarction (AMI) patients reveal the presence of isolated ST ↑ in posterior chest leads, the significance of this finding has not yet been determined. METHODS: We studied 33 consecutive patients with ischemic chest pain suggestive of AMI without ST ↑ in the standard ECG who had isolated ST ↑ in posterior chest leads V7 through V9. All patients had echocardiographic imaging within 48 h of admission, and 20 patients underwent coronary angiography. RESULTS: Acute myocardial infarction was confirmed enzymatically in all patients and on discharge ECG pathologic Q-waves appeared in leads V7 through V9 in 75% of the patients. On echocardiography, posterior wall- motion abnormality was visible in 97% of the patients, and 69% had evidence of mitral regurgitation (MR), which was moderate or severe in one-third of the patients. Four patients (12%), all with significant MR, had heart failure, and one died from free-wall rupture. The circumflex coronary artery was the infarct related artery in all catheterized patients. CONCLUSIONS: Isolated ST ↑ in leads V7 through V9 identify patients with acute posterior wall myocardial infarction. Early identification of those patients is important for adequate triage and treatment of patients with ischemic chest pain without ST ↑ on standard 12-lead ECG.