OBJECTIVE: The objective of this study was to examine the association between the age of having one's first child in adolescence and before marriage and religious involvement in Brazil, measured by religious affiliation and frequency of attendance at religious services or masses. The objective of this study was to examine the association between the age of having one's first child in adolescence and before marriage and religious involvement in Brazil, measured by religious affiliation and frequency of attendance at religious services or masses. METHODS: Transverse data obtained from the National Survey of Demographics and Health of 1996 and the National Survey of Demographics and Health of Women and Children of 2006 were utilized. Cox proportional risks models were employed to estimate the association between religion and age of having one's first child premaritally and during adolescence. RESULTS: The results indicate a strong association between premarital fertility in adolescence and religious involvement in both 1996 and 2006. In 1996, frequency of attendance at religious service s or masses was more important than religious affiliation in explaining the age at which one had her first child. In 2006, belonging to a Pentecostal church comes to predominate. CONCLUSIONS: The results presented in this study are encouraging insofar as they show that Protestant adolescents, particularly Pentecostals, have a reduced risk of adolescent premarital motherhood. This result was not expected, given that Pentecostalism predominates in the less advantaged population groups, with lower incomes and levels of education and residence in urban areas, where adolescent fertility is also concentrated in Brazil. Future studies must be undertaken with the purpose of understanding how the various mechanisms of religious influence operate in the life and behavior of adolescents in Brazil.