The serum concentration of immunoreactive follicle-stimulating hormone (FSH) and FSH response to synthetic luteinizing-hormone-releasing hormone (LHRH) were investigated in 9 patients with intact hydatidiform moles and 3 patients with aborted moles. The serum concentration of FSH before and after intravenous administration of 200 μg of synthetic LHRH was measured by double antibody radioimmunoassay. Seram FSH was detectable but relatively low compared with the levels seen in the follicular phase of the normal menstrual cycle in all 12 patients. Furthermore, the 12 patients did not respond, or responded subnormally, to exogenously administered LHRH, and there was no significant elevation in serum FSH levels after LHRH injection. The result of the current study is in accord with that of our previous report on the serum concentration of FSH and FSH response to synthetic LHRH during normal pregnancy. Thus, it might be concluded that the follicle-stimulating activity of the anterior pituitary is suppressed by an unknown mechanism during molar pregnancy as well as normal pregnancy.