We report a unique case of a 28-yr-old woman with a gonadotroph adenoma secreting FSH, presented with ovarian hyperstimulation, without elevation of serum estradiol. She presented with abdominal pain and large ovaries (both 10 cm in diameter) with multiple follicular cysts shortly after discontinuing oral contraceptive pills. She had a supranormal PRL level of 71 microg/liter (normal, <20), FSH of 8.4-9.2 IU/liter (normal for follicular phase, 2.4-10), LH of 0.01 IU/liter (normal, 1.6-9.3), estradiol of 108 pmol/liter (normal for follicular phase, 80-790), and free alpha-subunit level of 0.11 microg/liter (normal, <1.8). A nuclear magnetic resonance study revealed invasive pituitary macroadenoma, 30 mm in diameter. Dopamine agonist (cabergoline) treatment normalized serum PRL but had no affect on FSH levels. A transsphenoidal surgery was performed, and most of the adenoma was resected. One month after surgery the patient resumed menstruation, and the hormonal profile included serum FSH of 6.3 IU/liter, LH of 2.1 IU/liter, estradiol of 156 pmol/liter, and PRL of 10 microg/liter. The excised adenoma tissue exhibited intense immunostaining for FSH and secreted this hormone to culture medium. Stimulation with TRH (both in vivo preoperatively and in vitro study of the excised tumor) had no effect on FSH secretion from the adenoma. Estradiol did not suppress FSH release from cultured adenoma cells. Patient serum samples showed significant FSH bioactivity when tested in a human granulosa cell line. This case is remarkable because the ovarian hyperstimulation related to the FSH-secreting adenoma was not associated with high levels of serum estradiol, probably due to insufficient LH production by the normal pituitary. Thus, it supports the two-cell, two-gonadotropin theory, that both FSH and LH are necessary for normal ovarian estrogen production.