A breast mass developed in a patient receiving estrogen therapy for prostatic cancer. The prostate tumor was adenocarcinoma of small acinar type, whereas that of the breast was infiltrating medullary adenocarcinoma. Histological features of the two tumors differed and double cancer was suspected by conventional pathological study. However, immunohistochemical staining with prostatic specific antigen and prostatic acid phosphatase was positive in each tumor. These results indicate the breast tumor to be a metastasis from the prostatic cancer.