Ovarian hyperstimulation caused by a gonadotroph adenoma in premenopausal women has been described only twice before this report. A 28-yr-old woman presented with menstrual disturbances and pelvic pains that began after stopping the use of contraceptive pills. Transvaginal ultrasound revealed enlarged ovaries with multiple cysts. The patient had elevated serum estradiol (up to 2900 pmol/L; normal, 80-300 pmol/L in the follicular phase) and inhibin (6.4 kU/L; normal, 0.5-2.5 kU/L) levels. Serum LH was appropriately suppressed (0.6 IU/L), but serum FSH varied from 4.9-8.1 IU/L. Both gonadotropins as well as the free alpha-subunit showed a paradoxical response to the stimulus by TRH. A nuclear magnetic resonance study unraveled a pituitary tumor, 12-14 mm in diameter, extending up to the suprasellar cistern. After pituitary surgery, all hormone values normalized, and the patient resumed regular ovulatory cycles. In immunostaining, 20-30% of the cells of the tumor stained positively for FSHbeta. We conclude that a gonadotropin-producing adenoma must be considered in the differential diagnosis of a patient presenting with large multicystic ovaries and high estradiol levels in the absence of exogenous gonadotropins.