The genetic origin of hydatidiform moles was analysed utilizing HLA-DNA typing. Using HLA-DR type-specific oligonucleotide probes, the DRB types of seven moles were determined and compared with the parental DRB types to determine the paternal and/or maternal origin of the moles. In four cases, the molar tissues showed single DRB types of paternal origin, although in one, the molar DRB type was also possessed by the mother. These four moles were, therefore, considered to be androgenetic in origin. Chromosomal karyotyping was carried out for three of these cases and confirmed the DR-DNA typing results. Two moles demonstrated a DRB-type triplet, which strongly suggested triploidy. Although one mole showed a heterozygous DRB type, karyotyping indicated triploidy (69, XXX) and suggested that this mole was caused by dispermy-fertilization, in which both of the sperms had the same DRB type. Although the majority (about 80%) of partial hydatidiform moles have been reported to be triploid as a result of dispermy, four of the moles analysed in this study (cases 1, 2, 3 and 4), diagnosed as partial macroscopically and/or histopathologically, were found to be androgenetic in origin using karyotyping and DR-DNA typing. Therefore, HLA-DR DNA typing, combined in some cases with karyotyping, provides an accurate method for diagnosing androgenesis and triploidy in complete and partial hydatidiform moles.