Aims Successful epicardial reperfusion with primary percutaneous coronary intervention (PCI) for ST-elevation myocardial infarction (STEMI) can paradoxically evoke myocardial reperfusion injury, which may be signalled by temporally associated ventricular arrhythmias (VAs). We correlated reperfusion VA 'bursts' with final infarct size (IS) in patients with restored TIMI 3 flow following PCI for anterior STEMI. Methods and results All 128 anterior STEMI patients with final TIMI 3 flow had continuous 24 h digital 12-lead ECG with simultaneous Holter recording initiated prior to PCI, and Day 7/discharge SPECT imaging IS assessment. Angiography, SPECT imaging, continuous ST recovery, and quantitative rhythm analyses were performed. Reperfusion VA bursts were defined against patient-specific background VA rates and timed as concomitant with or following first angiographic TIMI 3 flow restoration associated with > or =50% stable ST recovery; they were then correlated with IS and global left ventricular ejection fraction (LVEF) at Day 7/discharge. Bursts occurred in 81/128 (63%) patients and were significantly correlated with larger IS and worse LVEF (median: 21.0 vs. 10.0%, P < 0.001; 35.5 vs. 46.5%, P < 0.001, respectively). In multivariable analyses that adjusted for known predictors of IS, the association of bursts with larger IS remained significant; similar results were seen for worse LVEF. Conclusion Reperfusion VA bursts predict larger IS despite TIMI 3 flow restoration with > or =50% stable ST recovery following PCI for anterior STEMI. Well-characterized reperfusion VAs may provide a novel biomarker of reperfusion injury.