Hydatidiform mole is a benign trophoblastic neoplasia characterized by an abnormal development of the embryo and proliferation of placental villi. Using microsatellite markers amplified by the polymerase chain reaction, we have performed a genetic study on eight independent molar tissues occurring in two sisters. Karyotype and genotype data demonstrate a diploid and biparental constitution in seven of the analyzed moles suggesting a common mechanism underlying the etiology of the various molar pregnancies in this family. The data reported here suggest that complete and partial hydatidiform moles are not always separate entities and that women with familial recurrent hydatidiform moles are homozygous for an autosomal recessive mutation.