In an elective study of 163 hydatidiform moles 38 were classified as partial mole (PHM) and 125 as complete mole (CHM) on the basis of pathology. Genetic studies showed the PHM to be triploid with one maternal and two paternal chromosome sets. In all cases of PHM the molar pregnancy resolved spontaneously after evacuation. On the basis of genetic studies CHM which were diploid could be subdivided into two entities: homozygous androgenetic CHMs that were 46,XX, and heterozygous CHMs which were androgenetic and usually 46,XY. In informative cases in this series the frequency of heterozygous CHM was 10 per cent. Twenty-two (17.6 per cent) of all the patients with CHM required subsequent chemotherapy for post-mole trophoblastic tumour. Where patients with CHM could be classified as having homozygous or heterozygous CHM the requirement for treatment (17.8 per cent and 25 per cent, respectively) was not found to be significantly different in the two groups.