Invasive trophoblastic disease could be divided genetically into three types depending on its origin as follows; both alleles of paternal origin, one paternal and the other maternal allele, and both maternal alleles. The last type is known as nongestational choriocarcinoma which is an extremely rare entity and has a poorer prognosis as compared to the other types. We studied a case of pure ovarian choriocarcinoma, which metastasized to the brain and lung during chemotherapy. From the pathological findings of trophoblastic invasion in brain and lung tissues, a remarkable affinity of the trophoblast cells for the vascular system was observed, explaining the hemorrhage around the tumor. The malignant trophoblast cells were analyzed by polymerase chain reaction amplification of DNA polymorphic alleles obtained at the resection of occipital metastatic trophoblastic tumor. The results revealed that the tumor cells were not androgenetic but completely from the host patient herself, and she was diagnosed to have nongestational pure ovarian choriocarcinoma. Since either allele was homozygous in certain loci and heterozygous in others, parthenogenetic origin of the trophoblast cells was suggested, more specifically that they might be deriving from a germ cell after meiosis. Following the aggressive treatment such as lobectomy of lung metastatic lesion and combined chemotherapy, the patient has survived for 6 years uneventfully, has normal hCG levels, and cyclic ovulation and menstruation. Genetic classification of malignant trophoblastic disease might provide important clues to the prognosis and type of clinical treatment required.