AbstractGastric bypass is an extensive exclusion operation which was developed in 1966 and has been used in over 600 patients for the treatment of morbid obesity. Stomal ulceration has developed in 2% of patients and has usually occurred because the stomach was transected at too low a level. To determine the effect of varying levels of transection and exclusion of the stomach, graded gastric bypass was studied in 2 peptic ulcer models in dogs. With histamine‐in‐beeswax, stomal ulcers occurred whenever there was more than 63% of the stomach located above the gastroenterostomy. With common duct ligation, stomal ulcers were not observed in dogs with 44% or less of the stomach excluded, but when more than 44% of the stomach emptied through the duodenum, duodenal ulcers developed. Stomal ulceration occurred as a result of excessive acid production from large fundic segments stimulated by histamine. Duodenal ulcers resulted from a deficiency in buffering of acid when sufficient stomach was excluded and when an adequate volume of acid entered the duodenum where bile was excluded. These experiments add further emphasis to the importance of high transection of the stomach in gastric bypass, but also suggest that the occurrence of common duct obstruction will introduce the risk of duodenal ulcer in patients who have had gastric bypass for the treatment of obesity.