Acerenza was in early imperial times a populous and important town, and a bulwark of the territory of Lucania and Apulia. In the Gothic and Lombard period it fell into decay, but was restored by Grimwald, Duke of Beneventum (687-689). An Archbishop of Acerenza (Giraldus) appears in 1063 in an act of donation of Robert Guiscard to the monastery of the Holy Trinity in Venosa.
For a few years after 968 Acerenza adopted the Greek Rite in consequence of an order of the Byzantine Emperor Nicephorus Phocas (963-969), whereby it was made one of five suffragans of the archdiocese of Otranto, and compelled to acknowledge the jurisdiction of the Patriarch of Constantinople Pope Urban VI (1378-89, Bartolommeo Prignano), was once Archbishop of Acerenza. The cathedral is one of the oldest and most beautiful in Italy, known for a bust long supposed to be that of St. Canus or Canius (Ascanius?) patron of the city, but now judged to be a portrait-bust of Julian the Apostate, though others maintain that it is a bust of the Emperor Frederick II, after the manner of the sculptors of the Antonine age.