This article reviews published data on familial recurrent hydatidiform mole with particular reference to the genetic basis of this condition, the likely outcome of subsequent pregnancies in affected women and the risk of persistent trophoblastic disease following molar pregnancies in these families. Familial recurrent hydatidiform mole is characterized by recurrent complete hydatidiform moles of biparental, rather than the more usual androgenetic, origin. Although the specific gene defect in these families has not been identified, genetic mapping has shown that in most families the gene responsible is located in a 1.1 Mb region on chromosome 19q13.4. Mutations in this gene result in dysregulation of imprinting in the female germ line with abnormal development of both embryonic and extraembryonic tissue. Subsequent pregnancies in women diagnosed with this condition are likely to be complete hydatidiform moles. In 152 pregnancies in affected women, 113 (74%) were complete hydatidiform moles, 26 (17%) were miscarriages, 6 (4%) were partial hydatidiform moles, and 7 (5%) were normal pregnancies. Molar pregnancies in women with familial recurrent
hydatidiform mole have a risk of progressing to persis-tent trophoblastic disease similar to that of androgenetic complete hydatidiform mole.