Adenosine is well known as a safe and effective drug for the termination of paroxysmal supraventricular tachycardia (PSVT), and is also widely used for the termination of both narrow and wide QRS-complex tachycardia of unknown origin in the setting of hemodynamic stability. However, due to a shortening of atrial refractoriness, adenosine can facilitate the induction of atrial fibrillation. A life threatening tachycardia may result from a potential rapid conduction of the atrial fibrillation over an accessory pathway. A case of patient, where the intravenous administration of adenosine, during regular, narrow QRS tachycardia, was followed by atrial fibrillation with rapid conduction over a manifest accessory pathway, is reported.