Abstract: Constitutional medicine was an influential medical science of the early twentieth century that attempted to classify individuals into different “human types” according to the presumption that bodily features and psychological traits were correlated to one another and mutually determined by the endocrine system. Although its purpose was to measure an individual’s health and susceptibility to disease, this dissertation argues for the significance of constitutional medicine as a transnational medical taxonomy of race, gender, and sexuality. By analyzing the medical research and professional connections of Gregorio Marañón (Spain), Agustín Cueva Tamariz (Ecuador), and Alejandro Lipschütz (Chile), this dissertation maps the circulation of constitutional medicine in the Iberian-American world and assesses its impact in particular national contexts. The study of socially marginal populations through constitutional medicine, such as inter-sexed individuals and indigenous groups, reveals broader concerns with defining national and regional identities in Spain and Latin America during this period.