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Five cases of a hitherto undescribed breast tumor having histologic features similar to those of the tall cell variant papillary thyroid carcinoma are described. They were composed of columnar mitochondrion-rich to oxyphilic cells arranged in nests, papillae, and follicle-like structures. In addition, the neoplastic cells showed numerous nuclear grooves and, in two cases, nuclear pseudo-inclusions. None of the patients had previous concomitant or subsequent evidence of a thyroid tumor. Immunohistochemistry further excluded a metastasis from the thyroid in the four cases tested, as they were consistently thyroglobulin and thyroid transcription factor 1 negative.