Two hundred two hydatidiform moles were classified by pathologic features and ploidy into partial or complete moles. Further classification was made by using genetic polymorphism, the diagnosis being refined by deoxyribonucleic acid technology. Among 51 partial hydatidiform moles, 44 triploids, two tetraploids, and one diploid were identified. Informative triploid partial hydatidiform moles had one maternal and two paternal sets of chromosomes, the likely origin being dispermy. Among 149 complete hydatidiform moles, one was haploid, one was triploid, and 105 were shown to be diploid or androgenetic; 39 of these were proved homozygous, indicating duplication of the male genome, whereas heterozygous origin by dispermy was likely in 13. The locus-specific minisatellite deoxyribonucleic acid probes were particularly useful for the identification of heterozygous complete hydatidiform moles. None of the patients with partial hydatidiform moles had development of a gestational trophoblastic tumor. No difference was detected in the frequency of requirement for chemotherapy between patients with homozygous or heterozygous complete hydatidiform moles.