After completing his novitiate, he obtained permission to visit the tombs of the Apostles and the Martyrs at Rome, and, upon his return, was made master of novices. After holding this office for ten years, he was elected superior. His zeal for strict religious discipline and for the purity of the clergy, as well as his deep humility and sincere piety, induced the General of his Order to entrust him with the foundation of two new Theatine houses, one at Milan and the other at Piacenza. By his efforts, many more Theatine houses rose up in various dioceses of Italy. As superior of some of these new foundations, he was so successful in converting sinners and heretics by his prudence in the direction of souls and by his eloquent preaching that numerous disciples thronged around him, eager to be under his spiritual guidance. One of the most noteworthy of his disciples was Lorenzo Scupoli, the author of "The Spiritual Combat". St. Charles Borromeo was an close friend of Saint Andrew Avellino and sought his advice in the most important affairs of the Church. He also requested him to establish a new Theatine house in Milan.