Twelve patients who underwent total colectomy and J-pouch-anal anastomosis were followed up to 3 years after surgery to evaluate the functional and morphologic changes of the small-bowel reservoir. Intestinal absorption was impaired for lactose in 18 percent, for D-xylose in 27 percent, and for 75SeHCAT in 83 percent. Morphologic changes in ileal-pouch specimens consisted of a marked flattening of the villi and augmentation in crypt number and length. The number of Paneth's cells was increased compared with normal ileum. Parameters, indicating neorectal function such as stool frequency, pouch volume, and intestinal transit, improved in time during the postoperative course. Because of impaired small-bowel function, which is not restricted to the pouch reservoir, the ileum acquires progressive colonic capacities in accordance with its morphologic transformation to a colonic type mucosa. None of our patients developed clinical malabsorption requiring regular therapeutic substitution beyond a well-balanced diet due to these morphologic and functional changes and postoperative acceptance was good or excellent in all but one case.