Objective: To determine the role of the hormone hCG in pregnancy, choriocarcinoma and cancer cell growth and invasion. Methods: Third trimester placenta cytotrophoblast primary cultures, two choriocarcinoma cell lines, and one trophoblastic cancer and 5 non-trophoblastic cancer cell lines. Results: Hyperglycosylated hCG promotes growth and invasion of normal pregnancy, 2 of 2 choriocarcinoma cell lines and 6 of 6 cancer cell lines. The hormone hCG attenuates or blocks growth and invasion in normal pregnancy, 2 of 2 choriocarcinoma and 6 of 6 cancer cell lines. Discussion: Cancer cells seemingly have adopted the hCG/LH receptor system. All cancers may be treated with hormone hCG to block and attenuate cancer growth, invasion and progression.