Three cases of metastatic adenocarcinoma of the male breast from prostatic carcinoma are added to the 15 well-documented cases reported in the literature. These 15 cases had received estrogen therapy for prostatic cancer and gynecomastia developed; 14 had clinically palpable breast nodules containing adenocarcinoma. Our 3 cases also received estrogen therapy but differed in that gynecomastia developed in only 1 patient clinically, and diagnoses were made at autopsy with no clinical symptoms related to breast metastases. Moreover, 1 cases also showed remarkable florid lactation-like changes of the breast almost indistinguishable morphologically from that seen in the female breast during pregnancy. The histopathologic differential diagnosis of metastatic prostatic carcinoma of the breast from primary cancer of the male breast is stressed. Its importance is obvious because of the differences in clinical treatment and prognosis. Microscopically, the differential points consist of duct hypertrophy and periductal fibrosis (gynecomastia), absence of any ductal involvement by carcinoma cells, frequent presence of cancer cells in lymphatics and vascular channels, morphologic similarity between the cancers in the breast and prostate, and finally, the usual presence of acid phosphatase in the tumors of the prostate and breast.