This study of the mariology of the letters of Augustine is part of an international project investigating the development of the cult of Mary before 431. The study argues that the works of authors before this date need to be considered individually, and that negative, ambiguous, or seemingly contradictory findings, as well as data in which Mary figures abundantly, are all valid. The scant role assigned to Mary in Augustine's letters, where she is mostly a credal commodity, stands in stark contrast to the high mariology found in some of his other works, indicating that genre affects the data. An aggregate score of high mariology in the one author, Augustine in this case, does not mean that this score holds true for all his works. A low score must be also taken into serious consideration, and indeed it can be just as important in understanding the development of the cult of Mary before 431.