Although primary breast cancer is common, metastatic disease to the breast, especially primary gastrointestinal cancer, is rare. Routine pathologic examination may be helpful in determining the true diagnosis, but can be misleading.To determine whether a signet ring carcinoma was a primary malignancy of the gastrointestinal tract metastatic to the breast or vice versa, histochemical analysis was performed for Her-2/NEU, gross cystic disease fluid protein-15, estrogen receptor, progesterone, carcinoembryonic antigen, cytokeratin 7, and cytokeratin 20.Positive staining for carcinoembryonic antigen and cytokeratin 20 (and negative staining for the breast cancer antigens), and the clinical criteria favors the diagnosis of gastrointestinal carcinoma metastatic to the mammary gland.Because the prognosis of therapy for metastatic cancer to the breast differs from that of primary breast cancer, it is imperative that the correct diagnosis be established. Immunohistochemistry for carcinoembryonic antigen and cytokeratin 20 are particularly useful. Metastatic gastrointestinal carcinoma to the breast is a rare lesion but needs to be at least included in the differential diagnosis of breast masses, especially in patients with a history of gastrointestinal cancer.