<p><i>NKX3.1</i> is an androgen-regulated tumor suppressor gene, and NKX3.1 protein expression is present mainly in prostatic epithelium. P501S is expressed in prostatic epithelium and is also regulated by androgens. Immunohistochemistry (IHC) for NKX3.1 and P501S is commonly used for the diagnosis of metastatic prostate carcinoma in pathology practice. Male breast cancer is rare, and most of the tumors are androgen-receptor positive. Thus, we investigated NKX3.1 and P501S expression in male breast carcinoma to determine its role in distinguishing primary breast carcinoma from metastatic prostatic adenocarcinoma. IHC for NKX3.1 and P501S was performed on primary invasive breast carcinomas in 25 male patients, 5 of whom had prior history of prostatic adenocarcinoma. Immunoreactivity was classified as negative or positive staining. Expression of NKX3.1 and P501S was identified in 8 (32%) and 7 (28%) cases of invasive breast carcinomas, 42% and 20% of in situ breast carcinomas, and 8% and 46% of normal terminal ductal lobular unit (TDLU), respectively. Among the five patients with prior history of prostate carcinoma, four had NKX3.1 or P501S expression in their primary invasive breast carcinomas. Their diagnosis of primary invasive breast carcinomas was confirmed by GATA3 and estrogen receptor (ER) IHC. This is the first study to report NKX3.1 and P501S expression in primary breast carcinomas in male patients. The positive rates are higher than those reported in female patients with ductal carcinoma. When considering the use of IHC markers to confirm the diagnosis of primary breast carcinoma in male patients, breast-specific markers or hormonal receptors should be considered in the diagnostic panel.</p>