OBJECTIVE: To examine the structure and metabolism of human chorionic gonadotropin (hCG) and the effect of molecular heterogeneity on the immunodiagnosis and monitoring of gestational trophoblastic disease. STUDY DESIGN: A review of the current medical literature concerning measurement of hCG and hCG-related molecules. RESULTS: hCG molecules in gestational trophoblastic disease are more heterogeneous or degraded in serum and urine samples than in normal pregnancy. CONCLUSION: Appropriate measurement and monitoring of hCG levels in gestational trophoblastic disease require an understanding of hCG structure and metabolism.