A total of 109 cases of carcinoma and 5 cases of adenoma of the ampulla of Vater were studied by preparing 5-mm step-wise whole tissue sections in each case. Carcinoma of the ampulla of Vater could be divided into four stages (I, II, III and IV), according to the microscopically verified extent of involvement by the tumor. All 12 patients with a Stage I tumor, defined as one restricted to within the muscle of Oddi as a boundary, showed no lymph node involvement and carried an excellent life expectancy with a relative 5-year survival rate of 85%. Thus, categorization of such a tumor as early carcinoma of the ampulla of Vater is valid. Excluding two unclassifiable cases, the remaining 95 patients with Stage II, III, or IV tumors extending beyond the boundary of Oddi carried relative 5-year survival rates of 11%, 25%, or 24%, respectively. Of the 109 carcinomas, 20 had areas of unequivocal adenoma at the margins, and of the five adenomas, two larger ones had foci of atypical epithelium suggestive of carcinoma in situ. These findings show the close relationship between adenoma and adenocarcinoma of the ampulla of Vater.