Sucrase-isomaltase (SI) is a mucosal disaccharidase that is present in normal small intestine and fetal colon. It also has been noted in colonic adenomas and adenocarcinomas. We used a polyclonal antibody to human SI to investigate enzyme presence and utility in detecting dysplastic changes in chronic ulcerative colitis. Sections from 32 cases were reviewed for the presence or absence of active colitis and dysplasia. Immunostaining of these cases for SI was performed and the results were reported based on location of immunoreactivity (ie, membrane and cytoplasmic staining in superficial and crypt epithelial cells) and percentage of positivity. Of 81 sections examined, 48 were rated negative for dysplasia (23 inactive colitis, 20 active, and five probably negative) and 28 were rated positive (eight low grade and 20 high grade). Surface membrane staining of epithelial cells was noted in all 28 dysplastic slides and positive cases (sensitivity, 100%) but also in 29 of 48 negative sections (P less than .001). In contrast, cytoplasmic positivity was present in 25 of 28 dysplastic and in only two of 48 negative slides (P less than .0001). The presence of cytoplasmic staining of SI in the superficial or crypt cells revealed a sensitivity of 92% and a specificity of 94%. There were five additional sections rated as indefinite for dysplasia (probably positive or unknown); two showed staining patterns typical of negative slides and three showed positive staining patterns. Of the 18 samples of transitional mucosa next to areas of dysplasia, surface membrane staining of SI was seen in all samples and cytoplasmic staining was seen in 15. We conclude that membrane staining of SI can be detected in inflammatory, regenerative, and dysplastic mucosa in ulcerative colitis. Cytoplasmic staining, however, correlates strongly with the presence of dysplastic change and may help in its detection.