Melo GB. Blastocystis sp.: molecular epidemiology and relationship with diseases in Brazilian scenarios. [thesis]. São Paulo: "Faculdade de Medicina, Universidade de São Paulo"; 2022. Blastocystis sp. is described as a protozoan prevalent in faecal samples from humans and animals, but still involves controversial aspects regarding its pathogenicity and epidemiology. In Brazil, there are few studies related to the understanding of relevant aspects of this enigmatic organism. Therefore, different approaches were proposed, such as the overview of research in Brazil in the last 20 years; the occurrence of Blastocystis sp. in the HC complex (Hospital das Clínicas) in the period of 2016 to 2021; the molecular characterization of isolates from patients with urticaria and diabetes, and perform in vitro cultivation of human isolates. The data of the literature review on the research of Blastocystis sp. in Brazil regarding the evaluation of its positivity in the HC-FMUSP complex reinforce the presence of this organism in our environment and raises the possibility of further studies to understand its real clinical significance. In relation to urticaria, it was not possible to establish a direct relationship with Blastocystis sp.Infection. Yet, it was possible to identify high intra and inter-subtypes diversity circulating in these patients. Regarding diabetics, although this protist was more prevalent in this group, it has not yet been possible to establish a relationship between diabetes and Blastocystis sp. Finally, our findings on the in vitro maintenance of human isolates allowed us to observed different cultivation times and forms present in different subtypes and alleles. Thus, further studies are needed focusing on the interaction between Blastocystis sp. and the host, to understand the impact of this organism on public health.