Abstract: Small intestinal bacterial overgrowth (SIBO) is defined as the colonization of fermentative bacteria in the duodenum and jejunum. The alteration of digestive anatomy promoted by bariatric surgery may be a pre-disposing factor for SIBO. In this context, the prevalence of SIBO in participants undergoing bariatric surgery using Roux-en-Y gastric bypass (BGYR) was evaluated.Participants, both sexes, older than 18 years, were those who (a) had bariatric surgery by the BGYR technique at least 1 year before the data collection and (b) did not use antibiotics recently. The SIBO diagnosis was established through the hydrogen breath test (H2BT), with intake of lactulose and serial collection of breath samples over 2 h. A test with >= 12-point elevation over the basal sample at 60 min after substrate intake was deemed positive. A total of 18 participants (14 females (77.8%)) were enrolled with a mean age of 50.5 years (range, 23 to 79 years). The interval between surgery and data collection ranged from 5 to 20 years (mean, 11.2 years). The mean preoperative body mass index (BMI) was 44.6 kg/m2 (range, 36.7-56.2 kg/m2). The H2RT with lactulose was positive for SIBO in seven (six female) participants. The participants with negative test measured trough H2BT with lactulose had a lower mean BMI of 28.69 kg/m2, in comparison with the positive group, which presented a mean BMI of 33.04 kg/m2 (p value?=?0.041).Our data point to a high prevalence of SIBO (38.8%) in patients undergoing BGYR with a value in accordance with the literature. Moreover, the differences in BMI between negative and positive groups by H2BT with lactulose evidenced a weight gain relapse in participants with SIBO Ver menos