Adenocarcinoid of the appendix is a tumor that shares the histologic features of both carcinoids and adenocarcinomas. The metastatic ability of the tumor is uncertain. Twenty consecutive cases of adenocarcinoids were compared with 88 cases of conventional appendiceal carcinoids. Metastatic tumor spread was found in six of 20 cases of adenocarcinoids, but in none of the conventional carcinoids. Metastasizing tumors were characterized by a moderate to severe nuclear atypia and a high mitotic count. These features were present in only two of the nonmetastasizing tumors, one of which was treated by hemicolectomy. Adenocarcinoids showing moderate or severe nuclear atypia, a mitotic count of two or more mitoses/10 high power fields, or spread beyond the appendix should be treated by hemicolectomy. If these features are not present, simple appendectomy is adequate.