We studied the effects of coronary recanalization on arrhythmogenesis in patients undergoing intracoronary thrombolysis during the early hours of myocardial infarction. Catheterization, ventriculography, coronary angiography, and intracoronary streptokinase infusion were performed in 22 patients. Twenty-one of 22 had thrombotic total occlusion of the infarct-related transient thrombolysis with reocclusion by the end of the procedure. In 12 of these 17 patients, restoration of antegrade coronary flow was accompanied by transient arrhythmia. In these 12 patients coronary angiography within seconds of onset of arrhythmia showed vessel patency in a previously totally occluded coronary artery. Two additional patients developed arrhythmias during streptokinase infusion but after reperfusion had already been established. Accelerated idioventricular rhythm was most often noted. Sinus bradycardia and atrioventricular block with hypotension occurred during restoration of flow in arteries supplying the inferoposterior left ventricle. These arrhythmias may be useful noninvasive markers of successful reperfusion during thrombolytic therapy in acute myocardial infarction.