We report a case of a pedunculated polyp with a focus of gastric-type adenocarcinoma arising in the opposite side of the papilla Vater of the duodenal second portion. The carcinoma was surrounded by lobules of hyperplastic Brunner's glands. Immunohistochemically the carcinoma tissue showed both gastric foveolar-type mucin ( MUC5AC) and pyloric/Brunner's gland-type mucin ( MUC6), in which proliferating cells positive for MIB-1 (Ki-67) were scattered diffusely. Most of the hyperplastic Brunner's glands were positive for MUC6, while cells toward the lumen in the superficial layer were positive for MUC5AC and MIB-1. This directional pattern of differentiation of Brunner's glands has recently been demonstrated by our group in the histogenetic course of gastric metaplasia originating directly from Brunner's glands. Therefore the present carcinoma is thought to have developed under induction of gastric-foveolar differentiation in a manner very similar to that of gastric metaplasia in hyperplastic Brunner's glands.