Metastases to the breast from extramammary primary malignancies, including renal adenocarcinoma, are rare. Fine-needle aspiration biopsy (FNA) is a useful, noninvasive, and rapid procedure to evaluate these mammary lesions. This study describes the cytomorphology of 3 cases of renal-cell adenocarcinoma metastatic to the breast. All patients had a prior history of renal-cell adenocarcinoma treated with radical nephrectomy, and they presented with a solitary mammary mass. The cytologic findings showed irregular clusters and dispersed single cells with eccentric nuclei and abundant, vacuolated cytoplasm in a hemorrhagic background. The nuclei were round to oval, with fine granular chromatin and a single, prominent nucleolus. All aspirates were interpreted initially and correctly as consistent with metastatic renal-cell adenocarcinoma. In summary, a cytologic diagnosis of renal-cell adenocarcinoma metastatic to the breast can be made by correlating clinical and cytologic findings. The distinction between metastatic extramammary malignancies to the breast and primary mammary carcinoma is critical to avoid unnecessary surgery and to ensure appropriate chemotherapy or radiation therapy.