The aim of our study was to determine if the rectal mucosa regenerates after rectal mucosectomy and endorectal ileoanal anastomosis for chronic ulcerative colitis. Such regenerated rectal mucosa could be the site of recurrent disease, leading to complications of the operation, and potential malignant degeneration. Pathologic specimens of the ileoanal anastomosis, surrounded by rectal muscular cuff, were obtained from eight patients who required takedown of their ileoanal anastomosis between one and 18 months after construction. Reepithelialization of the rectal cuff was not observed. In two patients, small islets of rectal mucosa and anal glands were identified. In all patients, the rectal muscularis propria was adherent to the serosa of the ileum by fibrous reaction. Three patients were diagnosed, both clinically and pathologically, as having chronic ulcerative colitis at the original ileoanal operation, but features suggestive of Crohn's disease were noted in the subsequently resected neo-rectum. Our observations suggest that, although isolated rectal mucosal cells may remain after mucosectomy, extensive rectal mucosal regeneration does not occur, thus minimizing the risk of recurrent disease and potential malignant change. Failure of an ileoanal anastomosis is therefore most likely related either to technical factors or to the presence of unsuspected Crohn's disease.