Gastric bypass as a 90 per cent gastric exclusion operation was used in 393 patients with massive obesity to limit food intake. Stomal ulcer has occurred in 1.8 per cent of such patients or one ulcer per 140 man years of observation. The studies of indwelling fundic pH and of gastric acid secretion from the excluded stomach indicate that acid secretion is reduced after gastric bypass but that the acid, unbuffered by food in the excluded stomach, results in a lowered gastrin secretion after a meal. Thus, gastric bypass in inhibitory to acid secretion in most morbidly obese patients who do not have known acid peptic disease.