Hydatidiform moles studied with respect to cytogenetics and morphologic constitution were divisible into two syndromes: (1) complete, classical mole giving a 46 XX karyotype and (2) partial mole with an ascertainable embryo/fetus, dead or alive, giving a triploid karyotype. The complete moles undergo early and total hydatidiform change from edema to central cistern formation, the embryos proper having perished before the establishment of a functioning circulation. Trophoblastic hyperplasia is conspicuous and the connection of this group to chorioncarcinoma is well established. In the partial moles there is a slow hydatidiform change that affects only some of the villi, but which seems to follow along the same lines as in complete moles. There is focal moderate trophoblastic hyperplasia, villous "trophoblastic inclusions" (that appear in triploids only), and maze-like central cisterns in the later cases. The partial mole, 46 XX, partakes of morphologic characteristics of both main syndromes and may represent an unusual syndrome of its own. The two main syndromes can now be distinguished morphologically and the question of the association of the partial mole with chorioncarcinoma has now to be further studied.