Cytogenetic and morphologic analysis of 23 hydatidiform moles allowed the division into at least two syndromes: (1) the syndrome of complete (classical) mole is without an ascertainable embryo/fetus, gives a diploid karyotype, and manifests a progressive fluid engorgement of the villi as well as a gross, haphazardly distributed trophoblastic hyperplasia; (2) the syndrome of partial (incomplete) mole has an ascertainable fetus (alive or dead), gives a triploid karyotype, and exhibits a slowly progressing hydatidiform swelling in the presence of functioning villous capillaries that spares many villi; trophoblastic immaturity is constant and focal hyperplasia is inconspicuous but present. A single case of diploid mole with unusual morphologic features, complete with a fetus, may herald yet another syndrome. Human chorionic gonadotropin levels were initially high in practically all cases. There was no malignant trophoblastic disease in this small series, but a plea is made that partial moles be followed carefully in order to establish their relation to choriocarcinoma.