An immunohistochemical study using PAb1801, a monoclonal antibody specific to the human p53 protein (wild and mutant), was performed to detect over-expression of p53 protein in colorectal cancers and dysplasia complicating ulcerative colitis, compared with that in sporadic colorectal cancers and adenomas. The over-expression of p53 was demonstrated in 6/7 (85.7%) cancers and in 15/18 (75.0%) dysplasias complicating ulcerative colitis. The frequency of p53 over-expression was 126/200 (63.0%) in sporadic cancers and 4/120 (3.3%) in adenomas. This study suggests that alteration of the p53 gene is implicated in the development of cancers complicating ulcerative colitis, as it is in the development of sporadic colorectal cancers, and it appears to be involved at a relatively early stage.