We describe a patient who was admitted to our hospital with an enlarged left lobe of the thyroid gland. Since fine-needle aspiration showed atypical follicular cells, a surgical exploration followed. Owing to extensive tumor infiltration into the surrounding tissues curative surgery was not possible, and only an incisional biopsy was taken. Histological examination of this biopsy revealed a mixed tumor composed of epithelial and myoepithelial cells. A primary thyroid tumor, metastasis of a salivary gland, and a skin appendage tumor could be excluded based on clinical examination, conventional histology, and immunohistochemistry. A tumor of the left breast treated 12 years earlier had originally been classified as an intraductal/intracystic carcinoma with focal invasion, but was re-examined. Using immunohistochemistry, the breast tumor was reclassified as a malignant adenomyoepithelioma. The current tumor was apparently a metastasis from this primary breast tumor. An updated review of the literature is given, including current knowledge on histological and immunohistochemical features of adenomyoepithelioma of the breast, with special attention to the reported pathological characteristics of recurrent and malignant tumors. Based on the reported pathological characteristics of recurrent and metastatic tumors we offer a diagnostic tool for identifying potentially malignant and recurrent tumors.