Choriocarcinomatous differentiation has been described in tumors arising from many organs including lung, rectum, colon, stomach, bladder, and rarely breast. Mammary carcinoma with choriocarcinomatous features is a rare variant of breast metaplastic carcinoma characterized by malignant cells morphologically resembling choriocarcinoma cells in which reactivity with human placental lactogen and human chorionic gonadotropin can be demonstrated by immunohistochemistry. The characteristic syncytiotrophoblast-like giant cells seen in these neoplasms are more commonly associated with moderately to poorly differentiated carcinomas with or without a clear-cut mesenchymal component. Most of the reported cases have behaved very aggressively. The reason for this poor prognosis remains unclear. Because of the small number of cases, special treatment protocols have not been developed and these patients are treated surgically and with the standard chemotherapeutic agents available for other types of carcinoma of the breast. Pathologically, these tumors must be distinguished from metastatic choriocarcinoma to the breast.