The aim of this study was to find out whether changes in the QTc interval duration in patients with acute myocardial infarction are related to the effectiveness of thrombolytic therapy. Forty-six patients were included in the study. Standard electrocardiograms were recorded on admission to the hospital, and 2, 12, 24, 48 and 96 hours later. The patients were divided into subgroups according to the location of acute myocardial infarction and results of thrombolytic therapy. ll subgroups showed a tendency of the QTc interval to shorten. The largest difference in the QTc interval duration was found between the first and last recording, yet in absolute QTc values, the difference was statistically significant only in the subgroup of patients with anterior acute myocardial infarction successfully managed by thrombolytic therapy (p < 0.02). A statistically significant decrease in the QTc value was also found in the subgroup with anterior acute myocardial infarction successfully treated by thrombolytic medication (p < 0.05). Although patients with anterior acute myocardial infarction successfully treated with trombolytic agents showed significant shortening of QTc intervals as compared to patients who failed to respond to the therapy, the difference in QTc intervals was in most cases of the same order of magnitude as the accuracy of measuring the QTc interval from a stancard electrocardiogram This fact, coupled with several other factors influencing the QTc interval in patients with acute myocardial infarction, limit the value of the QTc interval as an indicator of effectiveness of thrombolytic therapy.