Signal-averaged ECG has been used to identify patients at risk for ventricular tachycardia and sudden death after myocardial infarction. The goals of this prospective study were to examine the effects of reperfusion achieved with thrombolytic therapy on the 12-lead signal-averaged ECG and on ventricular arrhythmias in the early period after acute myocardial infarction (AMI). A total of 190 consecutive patients with AMI who fulfilled the inclusion criteria were enrolled. Thrombolysis was attempted in 80 patients and was considered successful in 57 (group I) and unsuccessful in 23 (group II); 110 patients were not treated with thrombolytic agents (group III). Signal averaging of 12 ECG leads was performed within 2 days in all patients and between 7 and 10 days after admission in 163 patients. The filtered QRS complex duration (QRSD) was significantly shorter in group I compared to group III in 7 of 12 ECG leads at 2 days and in 10 of 12 leads at 7 to 10 days. The root mean square voltage of the terminal 40 msec of the QRS complex (RMS40) did not change between the two signal-averaged ECG recordings in group I, whereas it became lower in three ECG leads in group II and in seven ECG leads in group III. There was no correlation between infarct site and significant changes in infarct-related signal-averaged ECG leads. The occurrence of complex ventricular arrhythmias was not significantly different among the three groups. We conclude that successful reperfusion, compared with failed and nonattempted reperfusion, is associated with fewer abnormalities in the 12-lead signal-averaged ECG in the early period after AMI.(ABSTRACT TRUNCATED AT 250 WORDS)