Localized giant pseudopolyposis is a rare complication in patients with ulcerative colitis which progresses to a huge intramural polypoid mass. Our case described here is a 30-yr-old female with chronic ulcerative colitis who developed localized giant pseudopolyposis with unexpected infiltrating adenocarcinoma. This case is one of an unusual form of cancer presentation in ulcerative colitis, and it indicates that we should be aware of the possibility of occult adenocarcinoma inside a large pseudopolyposis with no superficial dysplasia.