We studied reactivity of cytokeratins (CK) 7, 17, and 20 in 64 pancreaticobiliary adenocarcinomas to examine the effect of different cut-point thresholds on "positive" results, compare ampulla of Vater and pancreas adenocarcinomas, and provide additional experience with CK17 reactivity. Almost all neoplasms had extensive CK7 reactivity. The number of CK20-positive cases decreased from 29 (45%; any stained cells) to 19 (30%; > 25% staining) to 14 (22%; > 50% staining) with an increasing threshold of reactive cells. Similar shifts in the distribution of CK7 and CK20 reactivity occurred when different thresholds of reactivity were used for a positive result. There were no differences in CK7 or CK20 reactivity in pancreas only, ampulla only, and neoplasms involving both sites. Of 64 adenocarcinomas, 29 (45%) had no or single-cell CK17 reactivity, and 19 (30%) had reactivity in more than 50% of neoplastic cells. Ampulla of Vater and pancreas adenocarcinomas have similar CK immunophenotypes that cannot assist in distinguishing ampullary from pancreatic neoplasms on endoscopically procured tissue. CK17 staining occurs in approximately 50% of pancreaticobiliary adenocarcinomas and is usually patchy. Single antibody staining results, especially CK7 and CK20 coordinate reactivity, are influenced by the reactivity threshold used.