A child had two stagnant loops of the terminal part of the ileum, resulting from a bypass operation in infancy for ileal atresia. The patient had an iron deficiency anemia that was shown at operation to be due to mucosal ulceration within the terminal part of the ileum. Diffuse nodular lymphoid hyperplasia was also demonstrated within the blind loop, and the mucosal ulceration and nodular lymphoid hyperplasia may be a response of the ileal mucosa to the increased counts of aerobic and anaerobic bacteria and to deconjugated intraluminal bile salts. Deconjugated bile salts and bacteria with deconjugating properties were also found in the upper part of the small intestine, showing that lesions of the terminal part of the ileum may be associated with metabolic abnormalities within the upper part of the small intestine.