Immunostaining for thyroid transcription factor-1 (TTF-1) is frequently used to help assess the site of origin of metastatic carcinomas. TTF-1 expression is most frequently seen in carcinomas of thyroid and lung origin. Furthermore, it has been assumed that the expression of TTF-1 in a carcinoma excludes the possibility of a breast origin. We have recently encountered in our consultation practice 4 cases of invasive breast carcinoma (confirmed by clinical findings and other immunophenotypic features) that showed unequivocal tumor cell expression of TTF-1. However, the frequency with which TTF-1 expression is observed in breast carcinomas is unknown. To address this, we carried out immunostaining for TTF-1 on 546 primary breast carcinomas submitted for routine estrogen receptor, progesterone receptor, and/or HER2 testing. Cases were considered TTF-1 positive if they showed any nuclear staining for this marker. TTF-1 expression was identified in 13 cases (2.4%). Expression varied from focal and weak to diffuse and strong and was seen in both invasive and in situ components. We conclude that a small proportion of breast carcinomas show TTF-1 expression. Therefore, the presence of TTF-1 immunoreactivity in a carcinoma cannot by itself be used to exclude the possibility of a breast origin.