We identified three cases of tetraploid hydatidiform moles (HM). A complete hydatidiform mole (CM) had a 92,XXXX karyotype. The two partial hydatidiform moles (PM) had karyotypes of 69,XXY/90,XXXY,-11,-13 and 92,XXYY, respectively. Study of chromosomal heteromorphisms in the 92,XXYY PM revealed two maternal and two paternal haploid sets. Flow cytometric analysis of nuclear DNA content from fresh placental tissue from the 69,XXY/90,XXXY,-11,-13 PM demonstrated distinct peaks in the triploid and tetraploid regions and an octaploid G2/M peak. Flow cytometric analysis of paraffin-embedded, fixed tissue was diagnostic of tetraploidy in the 92,XXXX CM and consistent with tetraploidy in the 92,XXYY PM. All three patients achieved spontaneous remission of elevated gonadotropin levels. These three cases of tetraploid HM do not fit into the usual patterns of diploid CM and triploid PM. We conclude that flow cytometric analysis of nuclear DNA may be used to identify polyploidy in fresh and paraffin-embedded, fixed placental tissues. Categorization of all molar placentas on the basis of ploidy presents rare opportunities to study the biology and natural history of gestational trophoblastic disease in tetraploid HM.