: Expression of the mucin-associated sialyl-Tn (STn) antigen is correlated with malignant progression in the colon. In patients with longstanding ulcerative colitis (UC) who never exhibited colonic dysplasia, STn was expressed in fewer than 15% of nondysplastic surveillance colonoscopic biopsies, whereas matched patients who developed colon cancer expressed STn in >40% of previous surveillance biopsies, often several years before the first detection of dysplasia. Since no study has systematically examined STn expression in Crohn's colitis, we performed immunohistochemical staining with monoclonal antibody TKH2 on 110 surgical specimens from 59 patients with Crohn's colitis or ileocolitis. Forty percent of specimens expressed STn antigen. STn was preferentially expressed in specimens of the left colon, regardless of whether the patient had segmental left-sided disease or pancolitis. Specimens with active inflammation manifested increased STn expression, regardless of location in the colon or whether the patient had segmental disease or pancolitis. The enhanced expression of STn in Crohn's colitis compared with UC may reflect important differences in local cytokine profiles between the two inflammatory bowel diseases.