The aim of this paper is to present several basic features of the indebtedness of the kings of Poland to the clergy of the diocese of Cracow in the fifteenth century. This issue seems to be of importance for two principal reasons: firstly, on account of numerous wars waged by the kingdom of Poland in the fifteenth century, the rulers were often force to take to credit as a basic means of securing financial resources to cover the needs originating from the wars (reimbursement for knights’ merits, salaries for mercenaries, etc.); secondly, as opposed to earlier times, it was the rulers which became the most important debtors of clerical creditors in the period in question. The research is based upon the published source data concerning the clerical credit activity in the diocese. The first part of the paper presents several basic features of the rulers’ indebtedness to the clergy in the diocese in the Middle Ages: as opposed to the period 1251-1399 (the former date is marked by the first recorded example of clerical credit activity in the diocese), when the rulers appear on the third place among the debtors of clerical creditors, the kings of Poland become the most important customers of ecclesiastical money-lending in the fifteenth century. The second part deals in a more detailed way with the structure of ecclesiastical creditors in the period in question: unlike in the previous period, when it was feudal monasteries which dominated the clerical money- -lending, in the fifteenth century one notices a considerable rise in importance of other groups, like the bishops of Cracow, the Cracow cathedral chapter, and, generally speaking, urban clergy. This feature is of enormous importance from the point of view of the paper, as it was these groups which were chiefly involved in loans to the Crown. Regretfully, save the period 1440-1444, little (in quantitative terms) is known about the secular creditors of the kings: however, even these fragmentary figures allow to present some general data concerning the royal indebtedness as a whole: against this background, detailed features of clerical money-lending are dealt with. A difference in structure of clerical creditors to the Crown (as well as other features concerning technical details of credit activity) and clerical creditors to other social groups is underlined, with special reference to the town council of Cracow, which was a second most important money-borrower from the clergy in the period in question, and whose indebtedness is also strongly related to the political situation of the kingdom. Finally, a relative importance of the clerical credit to the Crown is estimated, by means of comparison of the volume of ecclesiastical loans at their peak point with the average yearly income of the Royal Treasury. The last part of the paper contains the conclusions: it underlines a correlation between the times of the greatest political hardship of the kingdom and the volume of ecclesiastical credit given to the Crown, it explain also the participation of particular groups of the clergy; moreover, it sheds light on the appliance of given types of credit. Finally, it is suggested that a comprehensive quantitative research concerning the royal indebtedness in the fifteenth century as a whole be carried out, which would allow the comparison of importance of particular social groups as money-lenders to the Crown.