The aim of the research was to analyse letters written by prisoners so as to discover what the penitentiary space means to them. The practical goal was to show the usefulness of employing epistolography in assessing the quality of social and living conditions and to evaluate the quality of penitentiary practices in the context of prisoners’ needs, as well as to unmask the hidden re-education programme. The article is based on the results of the content analysis of 26 anonymised letters written by prisoners who for the first time serve a sentence in Polish closed regime prisons. The collected research material was subjected to a framework analysis supplemented by an emergent technique. The analysis of prison letters indicates a generally critical assessment of the quality of the practices carried out and their accuracy in relation to the prisoner’s rehabilitation needs. The letters reveal also the existence of a hidden re-education programme. The use of the epistolary corpus in the description of the penitentiary space may result in the improvement of the current penitentiary methods. A thorough analysis of prison letters indicate areas that require change or the application of specific solutions that may contribute to the rationalization of penitentiary practices.