A middle-aged woman developed a postgastric bypass megaloblastic anemia which responded to treatment. She eventually had the bypass reversed 6 1/2 yr after it had been performed. Gastric parietal cell function has remained abnormal almost 3 yr after reversal of the bypass, as demonstrated by abnormal Schilling tests and high serum gastrin levels. Parietal cell antibodies in high titer, but no intrinsic factor antibodies, were demonstrated in her blood. These observations are interpreted as indicating the development of irreversible chronic atrophic gastritis probably related to reflux of bile into the bypassed stomach.