Bilateral breast mass was found in a 71-year-old male who had been placed on estrogen therapy for stage D2 prostatic adenocarcinoma. Microscopically the mass contained adenocarcinoma morphologically similar to that of the prostate, but the differential diagnosis was impossible between metastatic prostatic carcinoma and primary breast carcinoma. Formalin-paraffin sections of both tumors were stained positively by PSA (prostatic specific antigen) and PAP (prostatic acid phosphatase) using B-SA (biotin-streptavidin) system technique and prostatic origin of the breast mass was confirmed. Prostatic origin for metastatic carcinoma in the breast is are with only 30 reported cases in the literature including 5 Japanese cases. In most of them the diagnosis of the breast lesion as prostatic carcinoma has been made on morphologic and clinical grounds only. Accurate diagnosis is important for the prognosis of the patient, and immunohistochemical method is useful for he diagnosis of breast carcinoma metastasized from prostatic origin.