A radioimmunoassay (CTP-RIA) for urinary human chorionic gonadotropin (hCG) with the use of an antiserum to be carboxyl-terminal peptide of hCG beta subunit was employed to detect hCG production in patients with gestational trophoblastic disease. In urine samples obtained from normal subjects, the upper limit of hCG-immunoactivity detected by this assay system was 1.1 IU/24h. More than 90% of the subjects tested had values lower than 0.5 IU/24h. Based on these data, we selected urinary hCG levels below 1.1 IU/24 h as the normal range for clinical applications. The utility of this new assay system was assessed in 50 cases of gestational trophoblastic disease. In patients with hydatidiform mole, invasive mole and undetermined cases, the urinary hCG level declined to be normal range following the therapy and stayed there afterwards without any sign of recurrence. However, in a woman with a long history of metastatic choriocarcinoma, we noted the reappearance of hCG even after the hCG level once declined to the normal range. It therefore seems that cell viability will persist even in the normal range determined by CTP-RIA. Therefore, therapeutic decisions should take into account these points. This specific and sensitive CTP-RIA method for the detection of hCG production was found to improve the ability to diagnose persistent or recurrent trophoblastic disease.