Determination of human chorionic gonadotropin (hCG) is used for diagnosis and monitoring of pregnancy, pregnancy related disorders, for trophoblastic and some nontrophoblastic tumors. In addition, hCG is determined for doping control in males. Assay of hCG is complicated by the occurrence of different molecular forms, which are detected to various degrees by different assays. The main form of hCG in circulation and in patients with trophoblastic tumors is intact heterodimeric hCG. The free β subunit (hCGβ) is a minor form in plasma in both conditions, but it may be the major form aggressive trophoblastic cancer. Therefore, assays measuring hCG and hCGβ together are mainly used for diagnosis of pregnancy and trophoblastic diseases. When excreted into urine, most of hCG (and hCGβ) is broken down to the core fragment of hCGβ (hCGβcf), which is the main immunoreactive form of hCG in urine during pregnancy. Specific determination of hCGβ is of value in screening for Down's syndrome and diagnosis of nontrophoblastic cancer. hCGbcf is of limited utility but it is important because it may disturb assay of hCG in pregnancy.