BACKGROUND: The phase I KAT301 trial investigated the use of intramyocardial adenoviral vascular endothelial growth factor-D(DeltaNDeltaC) (AdVEGF-D) gene therapy (GT) to alleviate symptoms in refractory angina (RA) patients. In KAT301, 30 patients with RA were randomized to AdVEGF-D or the control group in 4:1 ratio. The treatment was found to be feasible, increasing myocardial perfusion and reducing angina symptoms at 1-year follow-up. However, there is some evidence suggesting that the intramyocardial delivery route and overexpression of (vascular endothelial growth) VEGFs might induce ventricular arrhythmias. Thus, we investigated whether intramyocardial AdVEGF-D GT increases the risk of ventricular arrhythmias in patients treated for RA. METHODS: We analyzed non-invasive risk predictors of ventricular arrhythmias from 12-lead electrocardiography (ECG) as well as heart rate variability (HRV) and the incidence of arrhythmias from 24 h ambulatory ECG at baseline and 3 and 12 months after the GT. In addition, we analyzed the incidence of new-onset arrhythmias and pacemaker implantations during 8.2 years (range 6.3-10.4 years) of follow-up. RESULTS: We found no significant increase in arrhythmias, including supraventricular and ventricular ectopic beats, atrial fibrillation, non-sustained ventricular tachycardias, and life-threatening tachycardias, nor changes in the non-invasive risk predictors of ventricular arrhythmias in the AdVEGF-D treated patients. Instead, we found a significant improvement in the very low and high-frequency bands of HRV suggestive of improved cardiac autonomic regulation after GT. CONCLUSIONS: In conclusion, our results suggest that AdVEGF-D GT does not predispose to arrhythmias and might improve HRV metrics.