OBJECTIVE: To investigate the determinants and prognostic value of heart rate variability (HRV) in acute myocardial infarction (AMI) patients treated by percutaneous coronary intervention (PCI). METHODS: We studied 64 patients with AMI (58.4 +/- 10 years, 56 men), treated by PCI within 12 h of symptom onset. Patients underwent 24-h electrocardiographic Holter recording before discharge. HRV was measured in the time- and frequency-domain. Cardiac events were assessed at 6 months. RESULTS: At univariate analysis, Thrombolysis in Myocardial Infarction (TIMI) flow grade < 3 was significantly predictive of reduced frequency-domain variables. On multivariate analysis, diabetes and pre-PCI time > 6 h were the strongest predictors of depressed HRV, with peak creatine kinase myocardial-type and lower left ventricular ejection fraction being also variably correlated with impaired HRV. At 6-month follow-up, seven patients (10.1%) had cardiac events (one AMI, six revascularization procedures). A low standard deviation of RR intervals was a significant predictor of events (P = 0.009), although only age was associated with the endpoint (P = 0.05) on multivariate analysis. CONCLUSIONS: Our study shows that, in AMI patients treated by primary PCI, a delay in coronary revascularization and AMI extension are key factors for determining depressed HRV.