We report the fine-needle aspiration cytology of a case of medullary thyroid carcinoma (MTC) metastatic to the breast in a 66-year old female within two years of diagnosis of the thyroid tumor. The aspirate of the breast metastases revealed a plasmacytoid population of cells in loose clusters and singly with mild to moderate pleomorphism. Nuclear groves and occasional intranuclear cytoplasmic inclusions were seen. The cells stained positive for calcitonin and negative for thyroglobulin. Use of immunocytochemical methods proved useful to diagnose metastasis which was essential in planning treatment. Cases of metastatic MTC to the breast diagnosed on fine-needle aspirates reported in the literature are reviewed.