An adenocarcinoid tumor of the duodenum, similar to those arising in the appendix, is reported. The tumor presented as a periampullary mass causing pancreaticobiliary obstruction. The microscopic features were typical of adenocarcinoid with diffuse infiltration of mucosa, submucosa, and smooth muscle by tubuloglandular structures lined by enteroendocrine and goblet cells. Signet ring cells were also present. It is desirable to distinguish adenocarcinoids from adenocarcinomas and typical carcinoids in this setting as, by analogy with appendiceal tumors, adenocarcinoids may be expected to have an intermediate prognosis.