Musicologists have already examined the relationships between Carlo Borromeo and music. However, the archbishop of Milan has been for a long time associated with a standard set of themes: actuation of musical decrees of the Council of Trent, intelligibility of the sung texts, discipline of church musicians and so on. In its first part, the present paper shows how this topic requires further investigation. The following part of the research takes into consideration printed materials produced during Borromeo’s episcopacy to examine his personal musical theology and to confirm that music was important and meaningful for him. In the last three parts of the work, attention is focused on Borromeo’s epistolary housed in the Biblioteca Ambrosiana of Milan: starting from these documents, it is possible to discover, for example, new musical connections between Borromeo and Nicolò Sfondrati, bishop of Cremona, or between Borromeo and Wilhelm V, duke of Bavaria; furthermore, these letters contain new details about Luca Marenzio and Giovanni Giacomo Gastoldi