Acute myocardial infarction (AMI) causing ST depression and T wave inversion has been diagnosed as subendocardial or non-Q myocardial infarction. However, some patients eventually develop strictly posterior infarction with a lesion of the left circumflex coronary artery (LCX). This study attempted to determine the electrocardiographic (ECG) characteristics of such myocardial infarction in 32 patients with definite AMI in whom ECG showed no hyperacute T waves or ST elevation and the LCX was an infarct-related coronary artery. ECG on admission (< 6 hours from the onset), at 24 hours, and on the 14th day were analyzed to evaluate QRS, ST, T, and U waves using calipers and magnifying lens. Sixty-six patients with normal circulation served as controls. The characteristic ECG findings on admission were ST depression in chest leads, and prominent positive U waves in leads V2 or V3 with relatively small T waves. Based on these results we proposed new ECG criteria: 1) ST depression > or = 0.1 mV in 2 consecutive chest leads, 2) prominent positive U wave > or = 0.1 mV in leads V2 or V3, 3) T/U ratio in leads V2 or V3 < or = 4. Considering two of the above criteria as positive, the sensitivity was 71.9%, the specificity 97.0%, and the diagnostic accuracy 88.8%. In 85.2% of the patients, ST depression returned to the baseline by 24 hours. As the amplitude of the U waves decrease gradually, the T/U ratio increased. The R/S ratio in leads V1 or V2 became > or = 1 by 24 hours in 46.4% and the amplitude of R wave in lead V1 increased gradually. T waves in the right precordial leads increased with time. These findings were consistent with isolated strictly posterior myocardial infarction. From these results we identified new ECG criteria: 1) R/S ratio in leads V1 or V2 > or = 1, 2) R wave > or = 0.7 mV in lead V1, 3) T wave > or = 0.5 mV in lead V1. Considering any of the above criteria as positive, the sensitivity was 72.0%, the specificity 87.9%, and the diagnostic accuracy 86.7% on the 14th day. These new ECG criteria of strictly posterior myocardial infarction with the LCX as an infarct-related coronary artery apply at less than 6 hours or at 24 hours from the onset of the symptoms.