Vagally mediated atrioventricular (AV) block is a condition which a paroxysmal AV block occurs with the slowing of the sinus rate. Owing to its unpredictability and benign nature, it often goes unrecognized in clinical practice. We present the case of a 49-year-old man who suddenly lost consciousness when he assumed a prone position for hemorrohoidectomy under spinal anesthesia; continuous electrocardiographic recording revealed AV block with ventricular asystole. He was completely recovered after returning to a supine position. This case calls our attention to fatal manifestation of vagally mediated AV block leading to syncope.