Two patients with a gastric hypersecretory state were studied over a 17- and 34-month period. In both patients, progressive morphologic lesions of the duodenum were noted. In one patient, the lesions initially consisted of inflammatory cells in the lamina propria, then of pyloric gland metaplasia, and finally gastric surface mucus cell metaplasia which was shown to involve the proximal jejunum. These morphologic changes were correlated with an abnormally low mucosal esterification of palmitate labeled with radioactive carbon and defective mucosal concentration of all aminoacids. Steatorrhea was present throughout his illness. In the second patient, morphological alterations of villous structure were not observed until the third year of observation, although mild inflammatory changes were seen earlier. Steatorrhea was intermittently present in this patient. However, the only clearly demonstrable alteration in mucosal function was in the concentration of basic amino acids.