This presentation deals with the large-scale statistics of 19 951 patients of trophoblastic diseases (TDs). The information was obtained by regional registration of 16 prefectures in Japan since 1974, and based on a definite classification of TDs given by the Japanese Society of Obstetrics and Gynaecology. The population of these prefectures comprises approximately 40% of that in all Japan. The number of registrations accumulated in this series was estimated to be greater than 90% of all affected patients in all regions, indicating a high degree of reliability of these statistics. The overall incidence of hydatidiform mole (HM) including invasive mole (IM) in 16 regions between 1974 and 1983 was one HM per 306 live births or 482 pregnancies. The incidence of HM with or without IM per 100,000 women showed a tendency to decrease in parallel with the reduction in birth rate during this period, thus indicating no significant change in the incidence of HM per 1000 live births. In this series the number of cases 'undetermined' because of a lack of appropriate histology was 943 (or approximately 5% of all cases). These may potentially be categorized as either IM or choriocarcinoma. Also, criteria for the distinction between a partial and a complete mole could not be defined satisfactorily.