Recognition of an ovarian tumor as a metastasis from an undiagnosed primary gastrointestinal tract carcinoma can be difficult when specific symptoms referable to the primary tumor are lacking and the tumor simulates a primary ovarian neoplasm grossly and microscopically. Ovarian metastases of colorectal adenocarcinomas, in particular, continue to pose diagnostic challenges both clinically and pathologically. Clinicopathologic features of 20 cases of ovarian metastases from undiagnosed colorectal adenocarcinomas (U-CRAs) were compared with those of 22 cases having metastases from known colorectal adenocarcinomas (K-CRAs). Women with ovarian metastases from U-CRAs were significantly younger (mean age, 48 years; median, 47 years) than those with ovarian metastases from K-CRAs (mean, 61 years; median, 63 years) (P = 0.002), presented with clinical findings related to the ovarian metastases, often had elevated CA-125 levels, and lacked specific symptoms due to the colorectal carcinomas, which were diagnosed only at the time of intraoperative evaluation of the ovarian tumors. Mean/median ovarian tumor sizes (12.8/13.0 cm for U-CRAs; 14.1/15.8 cm for K-CRAs) and frequencies of bilaterality (45% for U-CRAs and 36% for K-CRAs) were not significantly different for the 2 groups; frequencies of clinically unilateral tumors of more than 10 cm were similar in both groups (30% for U-CRAs and 45% for KCRAs). Other features more commonly observed in ovarian metastases from U-CRAs included mucinous differentiation, extracellular mucin production, and some degree of cytokeratin 7 expression; endometrioid-like differentiation was more common in metastases from K-CRA, but a garland pattern of necrosis and the presence of a confluent glandular, rather than infiltrative, pattern of invasion were similarly common in both groups. In cases having ovarian metastases from U-CRA, the younger age of the women, uniform presentation as pelvic masses without symptoms referable to the bowel, elevated CA-125 levels, occasional presentation as a large clinically unilateral tumor, frequent mucinous differentiation, and frequent coexpression of cytokeratin 7 are features that can contribute to misclassification of these metastases as primary ovarian neoplasms.