Lobular carcinoma of the breast is known to metastasize to unusual sites such as the gastrointestinal tract, peritoneum, and gynecologic organs. We report a patient with intraperitoneal metastases from lobular carcinoma who was originally treated for an unknown primary cancer. Ten years later, a tumor was found in her left breast and the diagnosis was changed to peritoneal metastases from invasive lobular carcinoma. Immunohistochemistry revealed that the metastases were high molecular weight cytokeratin (CK34betaE12) and estrogen receptor-positive, but were E-cadherin-negative. These results assisted in diagnosis. Surgeons should be aware of the characteristics of metastasis lobular carcinoma.