The malignant transformation of trophoblast is interesting to contemplate, as trophoblast normally behaves in a manner that is interpreted as indicative of malignancy. First of all, trophoblasts show "controlled invasion" at the placental site as part of the normal process of implantation. Secondly, it is estimated that 100,000 syncytiotrophoblastic cells are deported to the maternal circulation daily and these are commonly identified in the pulmonary circulation of pregnant women (38). These trophoblastic cells do not ordinarily produce disease and presumably are rejected by the mother, unlike true metastases. When trophoblastic malignancy does develop, however, the trophoblast continues to invade and grow without limit, eventually metastasizing and ultimately leading to death. Choriocarcinoma is, therefore, unique in that it represents a malignant transformation of a tissue that inherently has "invasive" and "metastatic" properties. It is also the only tumor which contains DNA foreign to the host, as it is derived from a conception which contains paternal genetic material. Thus, choriocarcinoma is a complex neoplasm, and to study it, one must study and understand graft rejection, immunologic mechanisms and a multitude of genetic concepts in addition to the mechanisms of invasion and metastasis.