Paroxysmal atrio-ventricular (AV) block is a relatively rare form of bradyarrhythmia that may be caused by vagal reflex, intrinsic His-Prukinje system (HPS) disorder, or idiopathic mechanisms. We report a case with paroxysmal AV block and syncopal episodes that appeared only during intra-atrial reentrant tachycardia (IART) after an ablation procedure. Syncope did not occur under sinus rhythm with stable 1:1 AV conduction. An HPS disorder was proven in an electrophysiological study. It was suggested that paroxysmal AV block was induced via a tachycardia-dependent mechanism with an exacerbation of latent HPS disorder. The occurrence of the IART was only transient, and there was no recurrent syncope during one-year follow-up. Pacemaker implantation could be avoided. <Learning objective: Paroxysmal atrio-ventricular block is a relatively rare form of bradyarrhythmia and can occur only during atrial tachyarrhythmia. Indications for a cardiac implantable electrical device should be carefully considered if the attributed tachycardia is treatable.>.