The question examined in this chapter is whether medical and surgical interventions on secondary sex characteristics for the treatment of adults diagnosed with gender dysphoria in Catholic health care are morally consistent with Catholic teaching on the creation of the human person as either female or male and with the Catholic moral tradition. Consideration of this question will include: an overview of the status of medical and surgical interventions for the treatment of gender dysphoria; an explanation of Catholic teaching on the creation of the human person as a composite unity of body and soul and other central, ontological components of this teaching; an explanation of the status of sex in the body/soul unity; and an application of certain principles in the Catholic moral tradition, such as the principle of totality. The analysis of the ontological and anthropological components of Catholic teaching and the Catholic moral tradition will show that the biological distinction between the primary and secondary sex characteristics of the human person carries important ontological and ethical significance for the moral permissibility of medical and surgical interventions on secondary sex characteristics in the treatment of gender dysphoria. The analysis will also evaluate two ecclesial positions on this issue.