Arrhythmias are used as markers of coronary reperfusion after administration of thrombolytic agents. We studied the effects of coronary recanalization on the development of arrhythmias in patients receiving thrombolytic agents during the early hours of acute myocardial infarction. Acute cardiac catheterization, assessing perfusion of the infarction-related artery and identifying the arrhythmias, was performed within eight hours of onset of symptoms. Fifty-six of 67 patients (84 percent) studied had total occlusion of the infarction-related artery, 25 of whom had restoration of coronary flow during the 90 minutes after initiation of therapy; 31 had no evidence of reperfusion. Eleven patients had subtotal vessel occlusion that did not change appreciably during therapy, and 20 patients (30 percent) had transient arrhythmias. There were no significant differences in the frequency of arrhythmias: 36 percent of reperfused patients, 19 percent of nonreperfused, and 45 percent of subtotal occlusion patients. Only bradyarrhythmia was significantly related to the restoration of flow of the right coronary artery. Other rhythm disturbances, including accelerated idioventricular rhythm, did not correlate with a change in the perfusion status of the vessel. Bradycardia may be a useful marker for right coronary artery reperfusion. Arrhythmias in general are not specific for coronary reperfusion and should not be used as event markers.