Introduction: One of the major reasons identified for failure to lose weight in gastric bypass surgery is size of the anastomosis at the gastrojejunostomy. There is no standard technique for measurement of this anastomosis by endoscopy. Therefore, many treatment regimens may lead to ineffective intervention. This study was performed to identify the most accurate method to endoscopically measure the lumen diameter at the anastomosis to allow better management of these patients.Method: Subjects were asked to endoscopically measure a ring of known diameter in a standardized plastic model of the esophagus and gastric pouch using 4 commonly used endoscopic measuring techniques and a double channel endoscope. Subjects used visual estimation (VE), instrument reference (IR) to a biopsy forceps, an 18 mm esophageal dilating balloon (DB) as reference, and a 30 mm endoscopic ruler (ER) made from an ERCP guide wire tip. The 5 models (33, 27, 24, 18 and 13 mm) were presented in random order. Data was collected and maintained in a database.Results: Ten(9 surgeons, 1 gastroentrologist) subjects participated. Endoscopic experience was >1000 scopes for 4 subjects; 250-500 for 3; <100 for 3. The VE was the least accurate with an average diversion (AD) from the actual diameter of 6.25 ± 4.95 mm (24.20 %); followed by IR, 3.89 ± 3.05 mm (14.80 %); then the ER, 2.4 ± 1.9 mm (9.20 %).The DB was the most accurate with AD of 1.46 ± 0.9 mm (7.20 %). Of the 200 total measurements, only 8 (4%) were accurate, 142 (71%) underestimated the size, and 50 (25%) overestimated. Underestimation was noted in 82.5% (33/40) of VE and IR measurents; ER had only 60% (24/40) underestimation; DB underestimated only 40% (16/40) of the time .Overestimation was highest using DB 55% (22/40), followed by ER method 45% (14/40), then IR 15% (6/40); and lowest in VE 12.5% (5/40). Measurements of the largest model diameter (33 mm) were underestimated 98% of the time. In the smallest diameter model (13 mm), 16% of the measurements were underestimated; 2% were accurate; and 82% were overestimated.Conclusion: Endoscopic measurement of lumen diameter is very inaccurate. Underestimation is the most likely error in measurement. The larger the diameter the more likely it will be underestimated; and the smaller the diameter the more likely it will be overestimated. Endoscopists should avoid visual estimation and use a standard reference tool (dilating balloon) to measure anastomotic diameter. This will allow more effective intervention for clinical problems related to anastomotic size.