A rapid solid-phase radioimmunoassay (RIA) specific for human chorionic gonadotropin (hCG) has been used for the measurement of serum hCG activity in patients with molar pregnancy and gestational trophoblastic disease (GTD). Serum hCG regression as determined by the specific RIA method after evacuation of uncomplicated molar pregnancy was noted to occur over a longer duration of time than previously reported from this Center using a nonspecific RIA system which measures human luteinizing hormone (hLH) and hCG simultaneously. Therapy for proliferative trophoblastic disease was withheld after evacuation of molar pregnancy while the serum hCG level regressed normally, but was instituted when the serum hCG level rose or plateaued for more than two consecutive weeks. Serum hCG levels in patients requiring chemotherapy for GTD were also more accurately monitored with the specific RIA method than with the nonspecific technic. Therapy was based solely on the hCG titer rather than the subsidence of toxicity, as has been our practice in the past. As a result, the duration of hospitalization, total dose of drug required for remission, and toxic side effects were substantially reduced without sacrificing the effectiveness of chemotherapy.