An immunohistochemical method using Pab1801, a monoclonal antibody specific to the human p53 protein, was applied to detect p53 expression in colorectal cancer and dysplasia complicating ulcerative colitis. Of 20 tissue samples with dysplasia, six showed positive immunoreactivity. Archival paraffin-embedded tissue blocks from 21 colitic cancers were analysed; 11 showed positive immunoreactivity, compared with ten of 21 samples from matched sporadic colorectal cancers (P not significant). Previous data suggesting that colorectal carcinoma complicating ulcerative colitis has a reduced frequency of c-Ki-ras mutation compared with sporadic cancer have led to the hypothesis that different genetic lesions underlie colitic and sporadic colorectal carcinoma. The present results suggest that this is not the case with regard to p53 gene alterations.