Ileal pouch-anal anastomosis is the definitive therapy for ulcerative colitis that is refractory to medical treatment or that has developed neoplasia. Patients with this procedure are routinely followed using directed endoscopic biopsies to monitor for dysplasia in the rectal cuff, residual/recurrent ulcerative colitis, and nonspecific acute inflammation of the ileal pouch (pouchitis), which have different clinical management and outcomes. Thus, accurate localization of mucosal biopsies is crucial to a definitive histological diagnosis, but is complicated by overlapping clinical presentations of pouchitis and ulcerative colitis, post-surgical and inflammatory changes to the mucosa, and altered endoscopic anatomy, resulting in difficulty determining whether a mucosal biopsy is ileal or rectal in origin for both the endoscopist and the pathologist. We explored the utility of CD10 immunohistochemistry to aid diagnosis in this clinical setting by highlighting the enteric mucosa, based on previous studies showing its utility in brush border staining and in the diagnosis of microvillous inclusion disease. We found uniformly positive CD10 immunostaining in normal enteric mucosa, but variable loss of expression in the setting of active enteritis. Specifically, CD10 staining was lost in up to 10% of the mucosa in 1/12 ileostomies and 4/13 enteric anastomoses, in 10-80% of the mucosa in 9/10 cases of Crohn's ileitis, in 10-60% of the mucosa in 7/16 ileal pouches, and in 20-90% of the mucosa in 6/8 cases of backwash ileitis, usually in the presence of active inflammation. There was no CD10 expression by normal or diseased colonic mucosa. Therefore, while CD10 immunostaining identifies normal enteric mucosa with 100% specificity, negative staining does not definitively exclude small intestinal mucosa in the setting of active enteritis, a common condition in ileal pouch mucosa.