Résumé : Periampullary adenocarcinomas arise from pancreatic, biliary, or ampullary ductal epithelium. Their origin is difficult or impossible to discern by routine light microscopy of cytologic or small biopsy specimens. We used image analysis to describe the morphologic features of Feulgen-stained nuclei of biliary and ampullary adenocarcinomas and to compare these findings with those of pancreatic ductal adenocarcinomas. Surgically resected cases of ampullary adenocarcinoma (n = 7) and cholangiocarcinoma (n = 26) were selected on the basis of available formalin-fixed, paraffin-embedded tissue and diagnostic clinical data. Disaggregated nuclei were stained by the Feulgen reaction and analyzed using a digital image processor. Data from 15 morphonuclear parameters were assessed and compared with the results from the morphonuclear analysis of 22 pancreatic ductal adenocarcinomas. Multivariate analysis with canonical transformation of the data defined the ampullary adenocarcinomas and cholangiocarcinomas as occupying similar factorial distributions, whereas the pancreatic carcinomas were separate and distinct. Monovariate analysis identified seven parameters distinguishing pancreatic carcinoma from both ampullary carcinoma and cholangiocarcinoma, with P values < or = 0.001 and two others having P values < or = 0.01. Adenocarcinomas of ampullary or bile duct origin possess similar morphonuclear features described by image analysis. Image analysis provides a mechanism to discriminate adenocarcinomas arising from the bile ducts or ampulla from those arising in the pancreas.