Our understanding of gestational trophoblastic diseases has progressed considerably in recent years in terms of our knowledge of their cytogenetic origin, histopathology, and natural history. Advances in molecular biology have been applied to the study of gestational trophoblastic diseases, providing new insights into their pathogenesis. Molecular biologic studies have now clearly demonstrated that complete molar pregnancies result from fertilization of an anuclear, "empty" ovum. Improved understanding of the cytogenetics and biology of gestational trophoblastic diseases may well contribute to further advances in patient care.