The employment of castrati in the Byzantine Church can be traced back to the choirmaster Brison in the fourth century. Brison was called upon by John Chrysostom to organize the antiphonal hymn-singing in the patriarchal church. Since eunuchs were generally considered to be remnants of a pagan past, castrati are seldom mentioned in early Byzantine sources, but beginning in the tenth century references to eunuchs or castrati became more and more frequent. By the twelfth century all the professional singers in the Hagia Sophia were castrati. The repertory of the castrati is discussed and the question is raised whether the introduction of castrati to the Sistine Chapel was influenced by the employment of castrati in Italo-Greek cloisters.