ENThe Vilnius University Library is in possession copy of a foundation document for the Carthusians in Bereza. This document has been prepared in 1651. It was the first congregation of the Carthusian in Great Duchy of Lithuania. This was done by Kazimierz Leon Sapieha grant Lithuanian vice-chancellors. This document was drawn up in Polish. Casimir Leon Sapieha founded this monastery in his estate near the river Jasiołda and equipped with numerous villages. In addition, the founder has equipped the Carthusian fathers with numerous library. The first prior was Jan Hagen. The intention of the founder was to create a Sapieha’s family necropolis in Bereza. The communication maintained with the parent order led to the exchange of knowledge and experience. The foundation act presented is a source of knowledge about modern culture and society, but it is also a testimony to the history of the development of the eastern territories of the Polish–Lithuanian Commonwealth. Keywords: Casimir Leon Sapieha, Carthusians, Grand Duchy of Lithuania, Bereza, foundation.