The history of psychiatry has been arising higher interest in the last years. However, there is still a great scarcity of information regarding the history of this field of medicine, which is fundamental for a better understanding of the transformations of the psychiatric medical thought in our milieu. It is essential a profound knowledge of the concepts related to the traditions, changes and permanences of the past for the adequate understanding of the specialty's present and the prognosis for its future. It is relevant, in order to be situated in the current days, to know psychiatry's history of concepts and medical practice in Brazil, by focusing the historical context, the political movements and the possible influences that are external to our culture, which have interfered, along the time, for psychiatry to present as such in the current days. The current work aimed at delineating the medical-psychiatric conception between the decades of 1920 and 1950 about the mental disorders and their treatments in the state of So Paulo, besides emphasizing its possible influences over the medical-psychiatric thought of our days. It was used the qualitative methodology, by integrating two different areas of knowledge, Psychiatry and History. It is an appropriate method to incorporating meanings and intentions to social acts, relations and structures. The data production was accomplished by the survey and analysis of historical documents produced in the decades of 1920 up to 1950 about the formation of the discipline and the cathedra of Psychiatry of the Medical School of the University of So Paulo (FMUSP), the formation of the So Paulo League of Mental Hygiene, subordinated to the Brazilian League of Mental Hygiene (LBHM), of the formation and action of the Institution of Social Assistance to Psychopaths of the State of So Paulo, of the institutional life of the Juquery Hospital, and of the conception and beginning of the activities of the Institute of Psychiatry (IPq) of the Clinical Hospital of the Medical School of the University of So Paulo. We conclude that the thought about mental health in São Paulo, within the period delimited by the study, was constituted based on great influence of other fields of knowledge determining the directions of psychiatry as a medical specialty in our milieu