Carcinoid tumors of the ovary are rare neoplasms that may be primary or metastatic. Clinicopathologic features such as unilaterality and early stage favor a primary ovarian neoplasm but in the absence of other teratomatous elements it may be difficult or impossible to determine whether an ovarian carcinoid is primary or metastatic. CDX-2 is a marker of intestinal differentiation that has been proposed as a marker of midgut origin for metastatic carcinoids. Its expression has not been tested in ovarian carcinoids. Additional markers of potential help in defining the origin of a carcinoid include cytokeratin (CK) 20, CK7, and thyroid transcription factor (TTF-1), none of which have been studied in ovarian carcinoids. We evaluated the diagnostic utility of CDX-2, CK20, CK7, and TTF-1 as well as conventional clinicopathologic features in determining the site of origin in 26 ovarian carcinoids (16 primary and 10 metastatic from midgut). Non-neoplastic premenopausal ovaries (n=10) served as controls. All primary ovarian carcinoids were unilateral whereas only 3/10 metastatic carcinoids were unilateral. Multinodular growth occurred in 6/10 metastatic carcinoids but not in any primary carcinoid. The average size of primary ovarian carcinoids was 3.4 cm (range: 0.2-13.5 cm) versus 10.2 cm for metastatic carcinoids (range: 4-32 cm). Of the primary ovarian carcinoids, 12/16 were 3 cm or smaller whereas all metastatic carcinoids were 4 cm or larger. Teratomatous elements were present in association with 10/16 primary ovarian carcinoids, whereas none were present in any metastatic carcinoid. The primary ovarian carcinoid types were insular (n=6), trabecular (n=3), strumal (n=6, of which 5 were trabecular pattern and 1 was insular pattern) or mucinous (n=1). CDX-2 was not expressed in any cells in normal ovaries. Among primary ovarian neoplasms, there was diffuse nuclear CDX-2 expression in 4/6 insular, 0/3 trabecular, 1/6 strumal (1/1 insular pattern and 0/5 trabecular pattern strumal carcinoids), and 1/1 mucinous carcinoids. All metastatic carcinoids, except for two of mucinous type, were insular. CDX-2 was diffusely and strongly expressed in all 8 metastatic insular carcinoids and in both metastatic mucinous carcinoids. None of the metastases was trabecular in type but 12 primary hindgut or foregut trabecular carcinoids were evaluated and all were negative for CDX-2. None of the ovarian carcinoids expressed TTF-1, CK7, or CK20, except for the primary and metastatic mucinous carcinoids, all of which were CK20-positive. These results demonstrate that CDX-2 cannot be used to determine if a carcinoid is primary in the ovary or metastatic from the intestine as insular and mucinous types of either origin express this marker. Trabecular carcinoids of either origin lack CDX-2 expression. CK20, CK7, or TTF-1 do not have diagnostic utility in this context. Conventional clinicopathologic features (unilaterality, lack of multinodular growth, early stage, presence of teratomatous elements, and size 3 cm or smaller) are the most helpful findings in suggesting a primary origin for an ovarian carcinoid tumor.