An epidemiological study with the aim of establishing the incidence of hydatidiform mole, persistent trophoblastic disease and choriocarcinoma in Stockholm County was performed. Based on the regional cancer registry and hospital registers, the incidence for 1975-1988 was calculated for the number of deliveries as well as the total number of pregnancies. Of the molar pregnancies, 6% were treated with chemotherapy because of invasive mole or choriocarcinoma. Non-molar choriocarcinoma occurred in 1/33,717 deliveries. Difficulties in assessing the incidence of gestational trophoblastic disease are discussed.
