Objective: This study has the objective of analyzing the work of Monsignor Alvaro Negromonte, one of the pioneers of Sex Education in Brazil. He wrote a series of books focused on christian formation of youth. Advocate for education in chastity, presented this way of life in his book Sexual Education (1937), his best known and most famous work, which influenced catholic education in Brazil in the early decades of the twentieth century. Design and Method: Using historic bibliografy research, the authors located and examined this important book published by Negromonte that contains information and descriptions about sexual conception and attitudes considered ideals for Negromonte and the catholic church. Results: We verified that the Catholic Church, with conservative ideals, which challenged the secularism in republican educational field, giving way to the discussion of sex education in youth. Consequently, Negromonte points out the need of essentially christian sexual education for young people. This work shows that the consolidation of Sexology in Brazil occurred in the first decades of 20th Century not only from the action of doctors and educators that created extensive work divulged in books and communications in scientific associations, but also by action of catholic church. Conclusions: Despite the conservative aspect of the work of Negromonte, we can not help but note his fundamental driving feature about the issue of sex education for children and adolescents. Like others pioneers, Negromonte made part of a movement that promoted intense, organized, bold and still unknown today actions in sex education in the first decades of XX Century.