A 53-year-old man with a 22-year history of ulcerative colitis(UC) (pancolitis), had an ulcerating rectal tumor. Resection of the rectum and sigmoid colon was performed. Pathology showed an expansive ulcerating adenocarcinoma tumor (type 2) invading the adventitia against a background of UC in a resolving phase. Dysplasia was also found in granular and flat mucosa adjacent to the invading carcinoma. Immunostaining for p53 showed diffuse positivity in both the carcinoma and dysplasia, and also in the mucosa with indefinite dysplasia or no dysplasia neighboring the dysplasia and carcinoma. Mapping of neoplasms and the area with p53 protein overexpression showed that the grade of dysplasia increased as the lesion approached the invasive carcinoma and that the mucosa with dysplasia was surrounded by mucosa without dysplasia or indefinite for dysplasia, but with p53 protein overexpression. In some areas without dysplasia showing p53 overexpression, there was significant morphometric enlargement of the area and diameter of the nucleus p53 Immunostaining is a good marker for assessing the genetic alterations that precede histological abnormalities and for diagnosing carcinoma in UC. Objective findings such as p53-protein overexpression and morphometric values should be used to evaluate cytological abnormalities in UC, as well as in common colorectal cancer.