The registry for trophoblastic disease in Wakayama, Japan was formed and 342 cases were analyzed. The total number of trophoblastic disease was 342 during 6 years (1972–1977). The number of hydatidiform moles, invasive moles, choriocarcinoma and transitional moles was 278 (81.3%), 28 (8.2%), 18 (5.3%) and 12 (3.5%), respectively. The greatest number of hydatidiform moles occurred in the range 25–29 years inclusive. The percentage of patients with hydatidiform moles over 45 years of age was significantly high. The incidence of malignant complications (invasive moles and choriocarcinoma) of hydatidiform moles was 16.5% in 284 molar pregnancies. The frequency of malignant complications was remarkably high in women over 40 years of age. Thus the epidemiologic study of trophoblastic disease might turn up some important data and the importance should be emphasized.