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The composition of the gastrointestinal microbiota is associated with obesity. We hypothesized that the gut microbiota influences the outcomes of bariatric surgery.We aimed to analyze using oral swabs and stool samples the microbiota of patients with morbid obesity who were undergoing laparoscopic sleeve gastrectomy (SG).A university hospital in Poland.This prospective cohort study was conducted between November 2018 and June 2019. Participants underwent SG or no surgery (controls). Results were then analyzed as a group 1 (surgical participants who achieved a percentage of excess weight loss [%EWL]50%), group 2 (surgical participants who achieved a %EWL50%), and group 3 (nonsurgical controls). %EWL was measured 6 months following surgery. Before surgery, oral swabs were obtained and stool samples were provided. The endpoint was the composition of the gut microbiota.Group 1 comprised 19 participants, group 2 comprised 11 participants, and group 3 comprised 16 participants. No participants were lost to follow-up during the study. Participants in group 1 had an oral microbiota that was enriched in the phyla Proteobacteria, and Bacteroidetes. Their intestinal microbiota was enriched in the Proteobacteria. In contrast, the oral microbiota of group 2 was enriched in the Actinobacteria and the intestinal microbiota was enriched in the phyla Bacteroidetes and Firmicutes.The compositions of the microbiota of the oral cavity and large intestine are related to the weight loss achieved following SG.