From January 1, 1970 to July 1, 1976, 128 women had hydatidiform moles evacuated at the Los Angeles County-University of Southern California Women's Hospitals. Of the 121 patients with follow-up, persistent trophoblastic disease (TD) was diagnosed in 32 (26.4 per cent). Remission was achieved in all treated patients. The frequency of TD was significantly increased in pregnancies large for dates (47.8 per cent), with a uterus greater than 20 weeks' gestational size (45.0 per cent) or with theca-lutein cysts greater than 5 cm. in diameter (50.0 per cent). A normal regression curve for serum beta-human gonadotropin as measured by radioimmunoassay has been constructed as an aid to the early recognition of postmolar TD (invasive mole and choriocarcinoma).