In the duodenum, mixed exocrine-endocrine tumors exhibiting both neuroendocrine and glandular differentiations [cf. appendiceal goblet cell carcinoids (GCCs)] are rare. We present a Japanese case with a duodenal GCC that was found during pathologic examination of a gastrectomy specimen removed for gastric mucosal cancer. The tumor was widely distributed within both the first portion of the duodenum and the gastric antrum, although mucosal involvement was observed only in the duodenum. The tumor cells formed solid nests, trabeculae, or tubules, and some displayed a goblet cell appearance. They were immunoreactive against antibodies for both serotonin and somatostatin, and showed an argentaffin reaction (similar to a "midgut" enterochromaffin cell carcinoid). Ultra-structurally, the tumor cells had an amphicrine nature. Physicians encounter GCC in the duodenum only rarely, and its discovery may be incidental. Its diagnosis will be challenging and will require careful clinical and pathologic examinations.