Gestational trophoblastic neoplasia represents a biologic spectrum of tumors progressing from the hydatid mole, to invasive mole, and to choriocarcinoma. This progression is reflected in increasing degrees of aneuploidy in the respective lesions. Just as there is a natural tendency for the rejection of the trophoblast of a normal pregnancy culminating either in parturition or in spontaneous abortion, rejection of tumors of trophoblast occurs at any point in the progression of the disease spectrum. The unusual effectiveness of chemotherapy in trophoblastic disease may be related to this natural tendency to rejection. This tendency, in turn, is thought to derive from the genetic disparity between the maternal host and the tumor tissue of fetal origin, since the fetus possesses both maternal and fetal antigens.