The tetraspanin protein superfamily member KAI1 suppresses tumor growth and metastasis in animal models and is downregulated in various human malignancies. In breast cancer, KAI1 is preferentially lost in estrogen receptor (ER)-positive tumors. Interestingly, most ER-negative primary breast cancers retain KAI1 expression. This study aimed to evaluate whether or not KAI1 is downregulated during progression to metastasis of these carcinomas. Expression of KAI1, ER, progesterone receptor, c-ErbB2, and Ki67 was analyzed in tissue microarrays comprising a large collection of distant organ metastases from human breast cancers (n = 92) by immunohistochemistry. Results were compared with a previously characterized set of primary breast tumors (n = 209). Immunoreactivity for KAI1 was observed in one-third of the metastases and was associated with lack of ER expression (P = 0.005). The high frequency of KAI1-positive cases in ER-negative primary tumors was maintained in ER-negative metastases. Expression of KAI1 was also observed in MDA-MB-468 and SK-BR-3, two ER-negative breast cancer cell lines of metastatic origin. Moreover, a reanalysis of independent microarray gene expression data indicated maintenance of KAI1 mRNA expression in metastases from ER-negative breast cancers. Furthermore, in a series of matched pairs of mammary carcinomas and metachronous distant metastases, all metastases from KAI1-positive/ER-negative primary tumors were KAI1-positive as well. Collectively, these findings demonstrate that the expression of KAI1 is maintained during progression to metastasis in a large proportion of ER-negative mammary carcinomas. This has significant implications for the use of KAI1 as a clinical marker and the understanding of the metastatic process in human breast cancer.