It has been well documented that ampullary carcinoma arises from a precancerous lesion, but there have been few studies concerning changes at the molecular level during the adenoma-carcinoma sequence. In this study, p53 overexpression during the progression of carcinoma was compared and the relation between p53 expression and prognosis was analyzed. Ninety-four cases of adenocarcinoma of the ampulla of Vater were reviewed histopathologically and examined for overexpression of p53 protein using the DO-7 (mouse monoclonal; DAKO, Glostrup, Denmark) antibody. The correlation of p53 overexpression with the existence of adenoma, clinical stage, histologic grade, and overall survival was investigated. The proportion of p53-positive cases among normal mucosa, adenoma, early stage carcinoma (I and II), advanced stage carcinoma (III and IV), and metastatic lesion was 0% (0/94), 14.3% (6/42), 32.3% (20/62), 53.1% (17/32), and 63.3% (19/30), respectively. The existence of adenoma or histologic grade of carcinoma did not correlate with p53 overexpression. The carcinoma having adenomatous component was more common in early stages (54.8% in stages I and II, 25% in stages III and IV; p = 0. 006) and in well-differentiated carcinoma (p = 0.001). The existence of adenoma or p53 overexpression did not independently correlate with prognosis. In contrast, the p53 overexpressed group without adenoma showed a worse prognosis than the remaining patients (p = 0. 0006) and this trend was still demonstrable when the groups were compared stage by stage. In ampullary carcinoma, p53 abnormality occurs during malignant transformation from the adenoma and continues during the tumor progression in carcinoma. The clinical prognosis of de novo carcinomas with p53 overexpression was worse than that of the remaining patients.