The morphologic and cytoproliferative patterns of the duodenal mucosa of two adult patients, one of whom had a short bowel, were evaluated after more than 2 months of postoperative total parenteral nutrition and 2 and 12 months after the resumption of oral alimentation. Morphometric analysis was performed on routinely processed duodenal biopsies. Cell proliferation was evaluated by means of in vitro bromodeoxyuridine uptake. The results were compared with those obtained in five healthy controls. After parenteral nutrition, patients showed significantly lower villus height and crypt depth than those of controls and a normal bromodeoxyuridine labeling index. After 2 months of refeeding, villus and crypt returned to normal, and the labeling index was increased. After 12 months of oral refeeding, labeling index, villus height, and crypt depth were similar to those of controls. The patient with the short bowel showed a number of cells per unit length of villus and crypt significantly greater than those of the controls and of the patient who underwent shorter intestinal resection. In human duodenal mucosa, (1) hypoplasia develops after long-term total parenteral nutrition; (2) mucosal recovery occurs through an increased cell proliferation after oral refeeding; and (3) extensive small bowel resection determines the development of relative hyperplasia.