Hydatidiform mole has been known for its potential for malignant transformation and for its various chromosomal karyotypes. However, the relationship between histologic grading of hydatidiform mole and its future malignant transformation is still controversial. This study was undertaken to determine the cytogentic aspects of gestational trophoblastic neoplasia, especially of hydatidiform mole with respect to its malignant transformation. Cytogenetic studies were performed in 34 cases of hydatidiform mole, 2 cases of invasive mole, and 2 cases of choriocarcinoma. The results were analyzed comparatively using clinical histopathological and endocrinological (human chorionic gonadotropin titer) data. Among the 34 cases of hydatidiform mole studied, 26 cases were complete moles and the remaining 8 were partial moles with karyotypes being diploid (46, XX, 24, 46, XY, 2), and triploid (69, XXY) respectively. Two cases of XX mole among 26 complete hydatidiform moles developed distant metastasis during the follow-ups, suggesting transformation into choriocarcinoma; both cases showed 46, XX in karyotype and Grade III in histologic grading. Not one case of triploid partial hydatidiform mole transformed into malignancy. The karyotypes of the two cases each of invasive mole and choriocarcinoma were from neardiploid to hypotetraploid, and aneuploid cells were predominant in choriocarcinoma.