Background <p>The highly effective one-anastomosis gastric bypass (OAGB) is one of the most widely performed procedures in bariatric/metabolic surgery. However, concerns remain regarding the association of OAGB with bile reflux and long-term weight regain. The FundoRing modification of OAGB (FundoRing-OAGB) incorporates the excluded stomach in a primary fundoplication designed to limit reflux and weight regain by exerting an anti-dilatation effect on the gastric pouch.</p> Patients and methods <p>This comparative prospective post-hoc sub-study included 50 patients (25 per group) randomly selected from participants in a precursor randomized controlled trial (RCT). The sub-study aimed to assess and compare the anatomical integrity and volumetric parameters of the gastric pouch at 1 and 3 years using endoscopic ultrasound (EUS) in patients who had undergone OAGB with, or without, the FundoRing modification. Measurements included pouch diameter, volume, and wall thickness.</p> Results <p>At 3 years, patients in the FundoRing-OAGB group (<i>n</i> = 21) had significantly lower pouch diameters (2.8 ± 0.23&#xa0;cm vs. 3.41 ± 0.4&#xa0;cm, <i>p</i> = 0.014) and volumes (60.39 ± 10.04 mL vs. 87.93 ± 25.79 mL, <i>p</i> &lt; 0.001) compared to the OAGB group (<i>n</i> = 20). In the FundoRing-OAGB group, at year 3, pouch volume had increased 36.25% vs. an increase of 74.33% in the OAGB group (<i>p</i> &lt; 0.001). Wall thickness and pouch length did not differ significantly.</p> Conclusion <p>The FundoRing-OAGB modification exerted a significant anti-dilatation effect on the gastric pouch, as confirmed reliably by EUS. This effect may potentially correlate with better long-term weight maintenance; however, these clinical outcomes were not assessed in the present study. Large studies with long-term follow-up are needed to validate these findings and assess their clinical impact.</p> Trial registration <p>Sub-study of#NCT04834635; registered 8 April 2021</p>