Partial moles are either diploid (46 chromosomes) or triploid (69 chromosomes). In 35 cases ploidy was measured by flow cytometry for nuclear deoxyribonucleic acid quantitation, indicating approximate chromosome number. Six of the 35 were triploid (17%) and the balance was diploid (83%). No complications occurred in the triploid group, while five of 25 (20%) diploid cases with evaluable follow-up had nonfatal sequelae. Complications included persistent mole treated by recurettage and four cases requiring chemotherapy (human chorionic gonadotropin titer plateau, clinical metastasis, overt choriocarcinoma, and placental site trophoblastic tumor). A combined morphologic and genetic classification of partial moles is recommended for identification and risk assessment. Deoxyribonucleic acid cell cycle fractions of the diploid partial moles may be helpful in the identification of patients at high risk for complications. A proliferation index greater than 3.6 separated the cases with sequelae from most of the uncomplicated cases (sensitivity 100%, specificity 95%).