In 1291 Pope Nicholas IV ordered the union of the abbey with the Guglielmites, a reformed branch of the Benedectines, in order to give back strength to the abbey. However, after another period of decay in the 15th century, Pope Pius II annexed St. Antimus to the new diocese of Montalcino-Pienza (1462), whose bishop was Pius' nephew. The abbey decayed to the point that in the 19th century it was used as stable. In the 1870s the Italian state restored it.