The treatise 'De institutione regii pueri' originates from 1502 and remains one of the first pedagogical works of its kind written in Poland. The author is unidentified, although assumptions have been about Queen Elizabeth Habsburg, who might have written it together with an educated scribe, perhaps of Italian origin. 'De institutione' is composed of two intertwining strata: directives about the education and upbringing of a prince, and pertinent illustrations-examples. In contrast to the second stratum, the first has already been the object of interest of historians, despite the fact that the following part contains valuable material about the moral aspect of the life of the Jagiellons: opinions about Kazimierz the Jagiellon and all of his sons. A comparison of all the views contained therein demonstrates that they are, as a rule, particular sentences, and in rarer cases - longer fragments of the text; out of a total 34, 11 deal with Kazimierz the Jagiellon, six - describe Wladyslaw, six - St. Kazimierz, six - Jan Olbracht, two - Aleksander, two - Zygmunt and one - Fryderyk. The features ascribed by the treatise's author to members of the Jagiellonian dynasty include accessibility, generosity, royal majesty, piety, concern for upbringing the offspring, moderation in food and drink, and equanimity. An attempted assessment of the reliability of those opinions involved appraising the credibility of the presumed author and a confrontation of the exemplification stratum of the work with its didactic counterpart and non-source knowledge. The drawn conclusions made it possible to assume that the image of the Jagiellons depicted in the treatise is deformed due to its incomplete character, while those traits which it contains were determined by historical reality.