A letter, especially a private one, is one of the types of sources that are rarely kept in monastery archives. Its characteristic subjectivity often creates problems of interpretation, and its value for establishing a factual basis is limited. Nevertheless, it is an invaluable source of knowledge about the history of customs and ways of thinking. We present a selection of seventeenth-century letters found in the archives of the remaining Benedictine monasteries in Lithuania, from the vicinity of Vilnius, Nieśwież and Słonim. They present typical problems of new foundations and everyday life of monasteries at that time. The largest of them retained traces of economic problems, as most monasteries were founded without adequate security, in the hope of the generosity of future lay benefactors. In this way, sometimes even the greatest Benedictines of that time tried to make a living from this place and, as far as possible, purchased villages and farms. The correspondence presented here also illustrates the problems that sometimes arise when admitting candidates to the novitiate, especially the problem of ensuring their right to make a choice of their own free will, required by the Church for a valid oath. Finally, the letters give us a picture of the vicissitudes associated with periods of epidemics or war: the departure of the sisters from the monastery, the mother superior's concern for the scattered nuns, and in some cases the need to re-establish the foundation when there was no place to return to after the war.