To study the dynamics of signal-averaged ECG and late potentials (LP) in the first month of myocardial infarction (MI) and the impact of coronary reperfusion on them, examinations were made of 98 patients with primary myocardial infarction, in 69 of whom coronary reperfusion was achieved. LP was found to be detectable just in the first hours of MI, their stabilization (steady-state appearance or cessation) mainly occurred at day 3 of the onset. LP detection at this time allowed them to be predicted before the patients' discharge (70% sensitivity and 95% specificity, 82% predictive value in the first 24 hours of MI, LPs are characterized by more instability than those in the subsequent period of the patients' hospitalization. Thrombolytic therapy and coronary artery patency have no impact on the frequency of LP recording and parameters of signal-averaged ECG. No significant difference was found in the frequency of recording LP in anterior and inferior MI before discharge. The frequency of LP recording does not depend on the sex and age of a patient, the maximum creatine phosphokinase levels, and the presence of postinfarction angina pectoris and heart failure.