One of the most exciting and useful developments in modern oncology has been the recognition that certain biochemical tags can be used as cell markers in diagnosis, in monitoring the effects of anti-tumor therapy, and in follow-up to detect tumor recurrence. The measurement of chorionic gonadotropin (CG) in women with choriocarcinoma and/or gestational trophoblastic disease was the first test that could be relied upon entirely for this purpose. Furthermore, the amount of functional neoplastic tissue could be approximated by determining the production rate of this hormone which in normal physiologic conditions is only produced by syncytiotrophoblasts. The measurement of CG is now similarly used for the management of patients (both male and female) with choriocarcinoma originating in the tests and ovary. More recently a wide variety of nontrophoblastic malignant neoplasms have also been shown to produce CG, presumably from primitive tumor cells. Work is currently underway to ascertain the extent and potential usefulness of this observation in a wide variety of tumor systems, in the hope that improved tumor monitoring might be achieved.