Ethanol ablation via the coronary venous system (CVS) has been reported to be effective and safe as a novel therapy for ventricular arrhythmias, especially for the epicardial and intramural origin sites. Previous studies reported no heart block complications during retrograde coronary venous ethanol ablation, neither atrioventricular block (AVB) nor other types of conductive system block. N/A A 67-year-old female with symptomatic and drug-refractory premature ventricular contractions (PVCs) was admitted for ablation therapy. The PVC burden was 31.7%. The 12-lead ECG showed a PVC with LBBB, inferior axis with a precordial transition in lead V3, and QS pattern in lead I. The QRS duration of PVC was 143ms. The Q-wave ratio of lead aVL/aVR was 1.47. Endocardial mapping was performed using a 3.5mm open-irrigated-tip catheter (ThermoCool, Biosense Webster, CA, USA). The earliest activation time was -8ms and -10ms pre-QRS at the septal aspect of right ventricular outflow tract (RVOT) and the left coronary sinus (LCC), respectively (Fig 1A). Ablation with 35-40W, 45-50°C was attempted but no response. We considered it might be an intramural origin site. The CVS angiography was performed and the ablation catheter was positioned in LCC for reference (Fig 1B). The earliest activation time (-27ms) was mapped in a septal branch of the anterior interventricular vein (AIV) with a 3.3F quadripolar catheter and local pace mapping matched well. Selective angiography confirmed the quadripolar catheter was in the septal branch rather than AIV (Fig 1C-D). A balloon was used to deliver ethanal and prevent its backflow to AIV. 1ml 96% ethanol was infused slowly (Fig 1E-F). PVC was eliminated immediately as soon as the ethanol infusion. But we noticed the PR interval prolonged from 178 to 190ms. More ethanol was injected carefully to consolidate the effect but we stopped infusion after a total of 2.5ml ethanol because the ECG showed complete RBBB and PR interval prolonged further to 202ms. RBBB and I° AVB were persistent during the whole hospitalization for 3 days. However, at 1-month follow-up, the QRS morphology returned to normal and the PR interval shortened to 172ms (Fig 1G). 24-hour Holter showed no recurrence. Ethanol ablation via the CVS also might induce heart block complications due to the adjacent anatomical position with the conductive system, even though it might be reversible. Operators should be cognizant of the potential risk.