To obtain some useful pathologic indicators for predicting the prognosis in carcinomas of the ampulla of Vater, we analyzed 24 surgically resected ampullary carcinomas pathologically with immunohistochemistry of cancer-associated antigens. Pancreatic invasion, lymph node metastasis, and histology of the tumor were significantly correlated with poor prognosis (p less than 0.01), but the size or ulceration of the tumor did not significantly affect the prognosis (p less than 0.05). Immunohistochemically, diffuse positivity for anti-CA19-9 monoclonal antibody was demonstrated in 10 carcinomas and that for anti-carcinoembryonic antigen (CEA), in 10. Eight of them showed synchronously diffuse immunoreactivities for both antigens. Although there was no significant correlation between diffuse positivity for CA19-9 and pathologic factors, CA19-9-positive cases exhibited significantly poor prognoses (p less than 0.01). Diffuse positivity for CEA was correlated with pancreatic invasion (p less than 0.05) and poor prognosis (p less than 0.05). Immunohistochemical study of cancer-associated antigens may disclose some malignant potential of ampullary carcinoma other than that expressed in the morphology. Furthermore, because of the consistency of staining results, immunohistochemistry of cancer-associated antigens may also be useful in predicting preoperatively the prognosis of ampullary carcinoma in biopsied materials.