Gestational trophoblastic disease is a term that describes a group of tumors that share several characteristics as follows: (1) they arise in fetal chorion, (2) they produce human chorionic gonadotropin (hCG), and (3) they respond extremely well to chemotherapy. Although rare, they have received a disproportionate amount of attention because they were the first metastatic solid tumor to be cured using chemotherapy. Also, hCG was the first reliable tumor marker. Finally, because they arise in fetal tissue, they have the potential for a strong immune response against paternal antigens in the tumor. This potential for immunologic rejection was thought initially to explain the success of chemotherapy in this disease. The early detection of gestational trophoblastic disease is successful in patients who have had a hydatidiform mole as the pregnancy event that begins the process but unsuccessful in the early detection of the development of choriocarcinoma after a normal term delivery, abortion (spontaneous or elective), or ectopic pregnancy. Surveillance after evacuation of a molar pregnancy (whether complete or a partial mole) consists of regular evaluation of hCG production and the detection of metastatic disease. However, the development of gestational choriocarcinoma after term pregnancy or an abortion (no molar tissue can develop as a consequence of these pregnancies) is detectable only by signs or symptoms of metastatic disease in any of the many organs to which this tissue can spread. Unlike most staging classifications in gynecologic cancers, which are based on histologic findings and tumor location, the classification used in gestational trophoblastic disease stresses other features that are more useful for treatment selection. Both the National Institutes of Health and the World Health Organization classifications emphasize the importance of recognizing factors that predict the likelihood of a tumor responding to chemotherapy. Currently available treatment can cure all patients except those who are in the very high-risk group, which usually is characterized by metastasis to the brain or liver or a history of prior chemotherapy. Even in this category, approximately 80% of patients are curable.