The objective of the presentation was to assess changes of the troponin T level--a very sensitive indicator of cardiac muscle damage--in patients with acute myocardial infarction treated by fibrinolysis, and to assess differences in troponin T levels between patients with successful reperfusion and without reperfusion. The troponin T level was examined in 28 patients with acute myocardial infarction (AIM), incl. 22 (78.6%) where probably reperfusion occurred, as concluded from a maximal increase of CPK and CPK MB values within 12 hours. In six patients with a later rise of enzyme values treatment was considered as probably unsuccessful. In both groups of patients troponin T values increased, particularly markedly in the group with probable reperfusion, starting from the 3rd to the 18th hour after the onset of treatment, when the differences in troponin values between the two groups were statistically significant at the 1% and 1% level of significance. An early rise of the troponin T level suggests more rapid release of so-called free cytoplasmic troponin T in patients with successful reperfusion. From the 54th hour there is a statistically insignificant rise of troponin T level in the group of patients without probable reperfusion and it suggests a more marked release of so-called structurally linked troponin T in patients where reperfusion was not achieved. Consistent with these results, the mean maximal value of troponin T during the period of 3-24 hours after the onset of treatment is 2.37 times higher than a corresponding value during the period of 60-108 hours in the group with probable reperfusion and conversely in the group without probable reperfusion the troponin T value is 1.2 times higher during the period 60-108 hours after the onset of treatment.