Since the introduction of hormonal therapy for the treatment of metastatic prostatic adenocarcinoma, there have been 33 reports of metastases of prostate carcinoma to the breast. We report two cases of diethylstilbestrol (DES)-treated patients with metastatic prostate adenocarcinoma who developed breast masses. The lesions had infiltrative patterns simulating primary breast carcinoma. Immunoperoxidase stains, prostate-specific antigen (PSA), and prostatic acid phosphatase (PAP) were positive, identifying these cases as metastatic prostatic carcinoma to the breast. Differentiating primary from secondary tumors in these patients is difficult since there have been 10 reports of primary breast carcinoma occurring in DES-treated patients with prostatic adenocarcinoma. Their differentiation is important to direct appropriate therapy, and PSA and PAP immunoperoxidase stains are important in their correct classification.