Inactivating mutations have proven to be instructive in elucidating the role of FSH in human ovarian function. We performed a detailed reproductive endocrine evaluation of a patient with inactivating mutations in the FSH beta-subunit gene who was hypo-estrogenic and had LH excess. The patient underwent a pelvic ultrasound and overnight frequent blood sampling followed by a human chorionic gonadotrophin (HCG) stimulation test. One month later she received human recombinant FSH, followed 24 h later by a second HCG stimulation test. Despite a mean LH serum concentration and LH pulse characteristics typical for polycystic ovaries (PCOS), baseline and dexamethasone-suppressed free testosterone were low-normal. The administration of HCG led to minimal stimulation of 17-hydroxyprogesterone and androgens. The patient had multicystic ovaries containing follicles 3-5 mm in diameter and responded to FSH with prompt increases in estradiol and inhibin B. There were no clinical or laboratory consequences of LH excess in this FSH-deficient woman. These findings support the hypothesis that excessive LH stimulation alone does not cause ovarian hyperandrogenism. We also found that follicular development was present in the absence of FSH. These antral follicles had apparently developed normally, since estradiol and inhibin B increased promptly after FSH administration.