Medical historians have investigated the twentieth-century tuberculosis sanatorium as an idiosyncratic therapeutic tool and environment, while architectural historians have been interested in the development and prominent representatives of the building type. This chapter focuses on patients as active agents interacting with the sanatorium space. Relying on Michel de Certeau’s distinction between space (espace) and place (lieu), the chapter asks how the sanatorium space was used, modified, sensed, and remembered by (former) patients. The primary source material consists of written reminiscences, letters, diaries, and photographs related to and originating in Finnish twentieth-century sanatoria. The chapter argues that the experience of the institutional space formed a key part of the overall sanatorium and illness experience. Furthermore, it demonstrates that the interaction between the patient and the building was less one-directional and more multiform than commonly assumed. Patients described the sanatorium space frequently, and often in highly emotional and sensory terms, but the way they experienced the space was varied and individual. Although the sanatorium was a highly regulated space, patients could use it creatively. They modified the institutional space, often with the purpose of individualizing and de-institutionalizing it, and frequently used it in ways unintended and unforeseen by the medical and architectural authorities.