['Weight regain after Roux-en-Y gastric bypass (RYGB) is associated with reductions in health status and quality of life. We evaluated whether gastrojejunal stoma diameter is a risk factor for weight regain after RYGB.', 'We examined data collected over 4 years from consecutive patients referred to a tertiary care bariatric center for upper endoscopy after RYGB. We used linear regression analysis to determine the association between the gastrojejunal stoma diameter and weight regain. We applied a logistic regression model using clinical and endoscopic parameters to develop a prediction rule for weight gain after RYGB.', 'Among 165 patients included in our study, 59% had significant weight regain (≥ 20% of maximum weight lost after the RYGB) and 41% did not. The mean percentage of maximal weight lost after RYGB that was regained in the entire cohort was 30% ± 22%. Gastrojejunal stoma diameter was associated significantly with weight regain after RYGB surgery in univariate analysis (β = .31, P < .0001). This association remained significant after adjusting for several known or purported risk factors for weight regain (β = .19, P = .003). We developed a simple prediction rule for weight regain after RYGB using a 7-point scoring system that includes the gastrojejunal stoma diameter, race, and percentage of maximal body weight lost after RYGB; a cut-off score of 4 or more points had an area under receiver operating characteristic curve of 0.76 and a positive predictive value of 75%.'] Does gastrojejunal stoma diameter predict weight regain after Roux-en-Y gastric bypass?