The article presents three different problems related to the source analysis of the oldest hagiography of St. Hyacinth, written by Stanislaus, a lector of theology (14th century). In the first part of the article, the author analyses the relations be-tween two most complete manuscripts of The Life of St. Hyacinth and comes to the conclusion that one of them is a valuable witness to the text, while the second is a codex descriptus. The second part of the article is an attempt at proving that one of the chapters (chapter L) of The Life is a later interpolation added in the 14th century. The aim of the third part is to prove that when writing about the beginnings of the Dominican Order in Poland, Stanislaus drew upon a source missing today, to which Jan Długosz had access later.