We report an unusual case of a gonadotroph adenoma in a 34-yr-old woman, revealed by a dramatic rise in the plasma estradiol (E 2 ) concentration (26,800 pmol/L; normal, Ͻ370), with nonsuppressed FSH and LH levels (4.9 and 2.4 mIU/mL, respectively). The PRL level was 503 ng/mL. The testosterone and progesterone levels were 7 and 17 nmol/L, respectively. The levels of inhibin ␣, inhibin A, and inhibin B were increased compared to normal values in both the follicular (fp) and luteal (lp) phases of the menstrual cycle [inhibin ␣, 1986 IU/L (fp normal, Ͻ700; lp normal, Ͻ1650); inhibin A, 254 pg/mL (fp normal, Ͻ20; lp normal, Ͻ120); inhibin B, 246 pg/mL (fp normal, Ͻ150; lp normal, Ͻ30 lp)]. Pituitary magnetic resonance imaging revealed a huge pituitary adenoma. After transphenoidal surgery, the patient presented with pituitary insufficiency and diabetes insipidus. RT-PCR of the tumor tissue was positive for LH, FSH, ␣-subunit, and PRL. This case is of particular interest because 1) although the E 2 level was extremely high, the patient did not present with ascitis, suggesting that chronic elevated E 2 does not play a crucial role in the hyperstimulation symptoms; 2) the extreme rise in E 2 was related to the cosecretion of FSH and LH, confirming the two-cell two-gonadotropin theory; and 3) the rise in inhibin B is associated with FSH secretion, whereas the rise in inhibin A is probably due to luteinization. (J Clin Endocrinol Metab 83: 3450 -3453, 1998)