The purpose of this article is to analyze the records related to the diagnosis of degeneration (concept derived from Traité des dégénérescences, by Bénedict-Augustin Morel, published in 1857), based on documents from institutions for the insane according to the Assistência aos Alienados no Brasil program (Brazilian Assistance to Individuals with Mental Illnesses). The aim of this work, with a theoretical framework, is to assess interpretative asymmetries related to theories of degeneration at the beginning of the 20th century. Emphasis is given to the particularities of interpretation of the theory of degeneration in the Brazil, which may be attributed to the policies introduced by the newly settled Republic regarding racial mixing in the local society as a result of the trafficking of enslaved Africans before the abolition of slavery (1888). Intellectuals started to explain the Brazilian reality through race, which often led to the diagnosis of people’s degeneration of psychophysiological nature. However, according to a diachronic perspective, the term “degeneration” has had different meanings, mainly within the period covered by this article, during which Juliano Moreira, a doctor with black and white ancestry and a Kraepelin scholar, occupied the role of director of the Hospício Nacional de Alienados (National Institute for Individuals with Mental Illnesses), from 1903 to 1930.