The object of this study was to analyse ventricular arrhythmias occurring during intravenous thrombolysis for acute myocardial infarction with respect to ST segment changes on 24 hour Holter ECG monitoring initiated at the same time as thrombolytic therapy and on repeated 12 lead electrocardiogrammes. Forty-one patients in whom the infarct-related artery was patent at coronary angiography carried out 30.5 +/- 3.1 hours (< or = 24 hours in 59% of cases) after the onset of chest pain were included. The time to normalisation of the ST segment was defined as the interval between maximum ST elevation to a steady state and helped identify rapid (< or = 60 minutes, n = 13) from intermediate (60-180 minutes, n = 15) and slow (> 180 minutes, n = 13) reperfusion. The incidence of ventricular arrhythmias was the same in all three groups, except for prolonged ventricular tachycardias (> 15 complexes): 69%, 13% and 15% respectively (p = 0.002). The number of arrhythmias was greater when the ST segment changes were rapid than when they were intermediate or slow. This was true for ventricular extrasystoles (p < 0.05), accelerated idioventricular rhythms (p < 0.05), early (< or = 6 hours from onset of thrombolysis) accelerated idioventricular rhythms (p < 0.01) and ventricular tachycardias (p < 0.05). Therefore, the number of ventricular arrhythmias seems to be related to the speed of ST segment change, suggesting that more sudden reperfusion is more arrhythmogenic.