 The diocese grew over the centuries, gaining territories from the dioceses of Perugia and Orvieto, and incorporating what had been the diocese of Bomarzo.The latest known bishop of Bomarzo is recorded in the Roman synod of Pope Benedict VIII in 1015.Up to the time of pope Urban V, Montefiascone was part of the "Diocese of Castrum Balneorensis" (Bagnoregio's Latin name), but on 31 August 1369 it was separated out by Pope Urban and made the seat of a new diocese.