Intracoronary thrombolysis showed the chronological order of clinical electric and biological changes following the reestablishment of coronary flow. These changes make up the reperfusion syndrome; ST segment changes are part of this syndrome. They occur in practically all cases at the moment of reperfusion. The ST elevation may regress more or less rapidly or, on the contrary, increase transiently to a greater or lesser degree. When associated with other criteria of reperfusion-enzyme changes, arrhythmias, ST changes contribute to the indirect diagnosis of reestablishment of coronary flow. Rapid decrease in ST segment elevation is usually associated with a good myocardial outcome. The prognostic significance of transient increases in ST elevation--so called "reperfusion ischaemia"--is not fully understood, in particular its relationship to myocardial reperfusion injury. The myocardial prognosis after reperfusion may be the "biological" sum of cellular lesions due to ischaemia and reperfusion.