Surgical measures for the treatment of morbid obesity include gastric bypass of the stomach and duodenum. We endoscopically evaluated the bypassed segments in 51 patients three to 24 months after a standard gastric bypass procedure. Retrograde endoscopy was successful in 33 of the patients (65%). There was significantly more gastritis by endoscopic grading in the distal bypassed segment than in the proximal part of the stomach. The gastritis in the bypassed segment was associated with pooled bile in 97% of the patients. The severity of gastritis did not appear to be related to the time after surgery and was as severe three months after surgery as it was at 24 months. Histologically, the gastritis was nonconfluent, and often the histologic and endoscopic findings were at variance. There was intestinal metaplasia in biopsy specimens from the distal pouch in four of the 33 patients. The mechanism for the high incidence of gastritis is unknown, but may be related to decreased antral motility due to distention of the proximal pouch with food. These patients require close observation for changes in the bypassed segment of stomach that may occur as a consequence of chronic inflammation.