To our knowledge, prograde access to the left ventricle to perform a left-sided AV node (AVN) ablation has not been reported. We present the case of a patient with Noonan's syndrome and supravalve aortic stenosis who required aortic valve replacement with a mechanical valve. Due to chronic atrial fibrillation, a transvenous pacing system was implanted and transcatheter radiofrequency ablation of the AVN was performed. Attempts at right-sided AVN ablation were unsuccessful. Due to the presence of the mechanical aortic valve, a left-sided AVN ablation was performed successfully using the transseptal approach. The patient died suddenly 6 months after the procedure, and the histologic study of the heart is reported.