Objective: The signal-averaged electrocardiography (SAECG) is known to be a useful tool for extraction and analysis of low-amplitude signal components. We found SAECG may be useful for precise location of the site of the myocardial necrosis and assessment of the severity of impaired left ventricular systolic function of patients with ST-elevation myocardial infarct (STEMI) in the acute phase. Methods: High-resolution (1 MHz) ECG from 3 groups were collected: healthy controls (20), patients with anterior (17) and inferior STEMI (21). The three orthogonal leads X, Y, Z were synthesized from the 12 standard leads by known transformation. Synchronized averaging was carried out over hundred P-QRS-T intervals of each orthogonal lead. The resulting intervals of all subjects within a group were additionally averaged. The obtained X, Y and Z patterns, as well as the derived loops in the vectorcardiographic planes (VCG patterns) were studied for significant divergences. Results: The summarized analysis presenting the possibilities of QRS- and T-vector indicators for correct classification of patients with STEMI shows that the determined discriminators classify correctly 91.4% of the examined patients. The optimized set of QRS-vector indicators for discrimination between healthy controls and patients with inferior STEMI include angle a of the maximal vector in both the sagittal and the horizontal plane. These two indicators show as high predictive value as all QRS-vector indicators – 82.9%. The optimized combination of QRS-vector indicators for discrimination between healthy controls and patients with anterior STEMI includes amplitude of the maximal vector in the frontal and sagittal planes, angle a of the maximal vector in the sagittal plane and the area of the loop in the frontal plane. This optimized combination has a common mean percentage of correctly classified patients of about 91.9%. The accuracy for infarct zone localization is improved with optimized combinations involving together QRS- and T-vector indicators. The achieved common mean percentages of correct classifications are 94.6% (healthy controls-anterior STEMI), 92.7% (healthy controls-inferior STEMI) and 97.4% (anterior STEMI-inferior STEMI). The set of all QRS-and T-vector indicators of patients with anterior STEMI regarding 2D-echocardiographic ejection fraction shows very high correlation coefficient, reaching about 0.99. In contrast, we did not find significantly high correlation in patients with inferior STEMI. Conclusions: Both the signal-averaged orthogonal ECG and the synthesized on its basis VCG show markedly high sensitivity regarding location of ST-elevation myocardial infarct. The possibility for facilitated and fast performance of this examination in clinical conditions, including emergency, the lack of necessity of specially trained staff for carrying out the examination and interpretation of the results, as well as the very low prime cost, make this electrophysiological method very suitable for application in the routine clinical practice for qualitative and quantitative assessment of patients with acute coronary syndromes.