A huge aneurysm of the sinus of Valsalva with conduction disturbance as a consequence of infective endocarditis in Behçet's disease is reported. The aneurysm extended not only into the ventricular septum but also to the right atrium and ventricle, with a complicated cavity formation. We speculate that complete atrioventricular block occurred due to an enlargement of the aneurysm into the ventricular septum, leading to a direct conduction system injury. Preoperative echocardiography and aortography were insufficient to recognize the extent of the lesion; subsequent operative examination revealed the true size. At operation, it is important to understand the lesion dimensions fully in order that appropriate surgical procedures be performed.