Some sort of restriction of the pouch emptying is supported by many surgeons to allow a sustained weight loss through the use of a ring placed circumferentially around the gastric pouch. Most previous studies focused on the length of the ring, not the actual diameter of the gastric pouch outlet. This study aims to evaluate the association between the actual diameter of the gastric pouch outlet and the weight loss in obese patients submitted to silicon ring Roux-en-Y gastric bypass. We studied prospectively 257 patients that underwent silicon ring (65 mm) Roux-en-Y gastric bypass between July 2005 and August 2007. Upper digestive endoscopy was performed to measure the diameter of the gastric pouch outlet at 1 and 2 years with the aid of calibrated balloons. The diameter of the gastric pouch outlet ranged from 9 to 14 mm (mean = 11 mm). Excess weight loss ranged from 37% to 127% (mean = 69%) during the first year and 29% to 110% (mean = 69%) during the second year. A negative correlation between the diameter of the gastric pouch outlet and excess weight loss at first year (r = -0.792, p < 0.001) and at the second year of follow-up (r = -0.921, p < 0.001) was found. The actual diameter of the gastric pouch outlet was associated with weight loss after silicon ring Roux-en-Y gastric bypass during the 2-year follow-up.