A monk of Monte Cassino and chancellor of Pope Paschal II, Caetani was unanimously elected to succeed him.

His tenure also established the precedent of the papal chancellor always being a cardinal and holding the office for life or until elected pope.

He drove Gelasius II from Rome in March 1118, pronounced his election null and void, and set up Maurice Bourdin, Archbishop of Braga, as antipope under the name of Gregory VIII.

But the disturbances of the imperialist party, especially those of the Frangipani, who attacked the Pope while celebrating Mass in the church of St. Prassede, compelled Gelasius II to go once more into exile.

He was received with great enthusiasm at Avignon, Montpellier and other cities, held a synod at Vienne in January 1119, and was planning to hold a general council to settle the investiture contest when he died at the Abbey of Cluny.