Background Despite early recanalization of an occluded infarct-related artery, myocardial reperfusion may remain impaired due to microvascular injury. Reperfusion arrhythmias may indicate successful microvascular reperfusion. Methods Microvascular reperfusion was assessed prospectively in 42 consecutive patients with ST-segment elevation acute myocardial infarction (AMI) by evaluation of the resolution of ST-segment elevation (< 50% of initial level) immediately after successful coronary angioplasty. Patients were divided into two groups: those with ST resolution (n = 24) and those without ST resolution (n = 18). The presence of reperfusion arrhythmias immediately after recanalization was recorded. Results Patients with ST resolution were younger (54 +/- 12 years compared with 64 +/- 17 years, P = 0.04), their pain-to-recanalization time was shorter (195 +/- 87 min compared with 294 +/- 179 min, P = 0.05), they were less often diabetic (13% compared with 24%, P = 0.05) and were more often given IIb/IIIa inhibitors (58% compared with 22%, P = 0.02). Reperfusion arrhythmias were observed in 15 out of 24 patients with ST resolution (62%) but in only one out of 18 without ST resolution (5%) (P < 0.01). Reperfusion arrhythmias included accelerated idioventricular rhythm, 13 (81%); multifocal ventricular premature beats, two (13%); and ventricular tachycardia, one (6%). The sensitivity and specificity of reperfusion arrhythmias for ST resolution were 62 and 95%, respectively. In a logistic regression model including age, time to treatment, diabetes, use of Ilb/IIIa inhibitors and reperfusion arrhythmias, only the latter was found to be an independent predictor of ST resolution (P < 0.01). Conclusion Reperfusion arrhythmias following coronary angioplasty for AMI are a highly specific marker for ST resolution and may indicate successful microvascular reperfusion. (C) 2003 Lippincott Williams & Wilkins.