Metastases to the breast are rare, accounting for an estimated 1% to 2% of malignant breast neoplasms. The key histopathologic features supporting a metastasis to the breast have been stated to be the absence of elastosis, presence of a pushing border (circumscribed lesion), multiple satellite foci, lymphatic emboli, and, most importantly, the absence of an in situ carcinoma component. We report a unique case of a pancreatic islet cell tumor metastatic to the breast of an 18-year-old girl. Clinically, the patient was thought to have a mammary primary because on her initial biopsy, the metastasis grew within mammary ducts and colonized a complex sclerosing lesion, simulating an in situ component. However, review of slides from the prior pancreatic neoplasm, review of slides from the subsequent mastectomy, and use of immunohistochemistry allowed recognition of the lesion as a metastasis, which proved to be the first clinical manifestation of a systemic relapse. To our knowledge, this is the second case of islet cell tumor reported to metastasize to the breast, and the first report of a metastasis proven to have grown within existing ducts of the breast by immunohistochemistry.