Epidemiological investigation of the incidence and aetiology of hydatidiform mole (HM) is receiving increasing attention. Recent, population-based studies suggest that earlier reports of a very high incidence of HM in Asia, Africa and South-Central America may have been exaggerated, due primarily to selection bias in patients studied at university hospitals. Japanese population studies indicate a two-fold higher rate of HM compared with Caucasian rates but Chinese rates appear to be similar. Population studies presently available suggest a worldwide range of HM somewhere between 0.5 and 2.5/1000 pregnancies. When deliveries form the rate denominator the rates are somewhat higher, depending primarily on the national rate of induced abortions. The independent effects on incidence of geographic locale, ethnicity and socio-cultural factors have not been adequately disentangled although the genetic studies suggest ethnicity might be the predominant variable. Maternal age is the most consistently demonstrated risk factor; teenagers and, especially, women over age 35 being at increased risk. The independent effects of paternal age and pregnancy history are not established. Women with a history of one HM seem to have a ten-fold risk of repeat HM compared with women who have no history of HM. Aetiological studies have not revealed any environmental risk factor for which there is unequivocal agreement about its influence on HM. New case-control studies of HM aetiology must classify HM according to genetic aetiology. Cohort studies are required to explore more fully the relation of HM to malignant sequelae.