Recent reports on the incidence of hydatidiform mole and choriocarcinoma are reviewed, with consideration of the methodologic difficulties of obtaining accurate rates. The incidence of hydatidiform mole appears to be about 1 per 1,000 pregnancies in most parts of the world and perhaps twice as high in Japan. Choriocarcinoma is much less common, and estimates of the incidence are highly variable. Maternal age and a history of hydatidiform mole have been established as strong risk factors for both hydatidiform mole and choriocarcinoma. The evidence for a role of other factors, including diet, ethnicity, endogenous estrogen levels, ABO blood group and environmental toxins, is weaker.