This article examines the legislation allowing confiscation of the correspondence of the mentally ill in psychiatric hospitals. Arguing a duty of care, patients' letters were read by physicians and administrators. A study was performed of the regulations governing this practice in different Spanish institutions from the nineteenth century on; the measure was implemented by staff members under orders from their superiors. This arbitrary decision meant that a great deal of correspondence remains in the archives of psychiatric establishments in different locations; nowadays, these letters can be used as valuable clinical documents that help us to understand daily life in those institutions and, obviously, mental health patients' subjective experience of their confinement.