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To evaluate the importance of luteinizing hormone (LH) for normal estrogen production and subsequent development of ovarian follicles, a woman with isolated gonadotropin deficiency (LH; 0.37 IU/L, FSH 1.2 IU/L) was monitored during recombinant human follicle-stimulating hormone (hFSHrec) administration with respect to ovarian follicular growth and steroid production. During the first week (75 IU/day hFSHrec im) a significant rise in serum FSH (4.9 IU/L) was observed in the absence of changes in serum estradiol (E2) concentrations (36-76 pmol/L). During the following five days 150 IU/day hFSHrec was administered resulting in a further increase of serum FSH levels (maximum 8.5 IU/L). Development of multiple follicles--maximum diameter 22 mm as observed by transvaginal sonography--emerged together with a minor rise in E2 levels (from 76 to 236 pmol/L) and with a minimal increase in endometrial thickness (below 6 mm). Six days following the last injection of hFSHrec, aspiration of 3 follicles (13, 15 and 18 mm) was performed and low intrafollicular androstenedione (AD) (less than 675 nmol/L) and E2 (less than 9400 pmol/L) concentrations as compared to normal follicles were found. These first data on hFSHrec administration in the human suggest that; a) FSH alone can induce growth of preovulatory follicles, b) follicle growth does occur in the presence of subnormal E2 levels, c) LH is needed for adequate AD biosynthesis as substrate for aromatase activity. This indicates that growth and steroidogenic granulosa cell activity may be differentially regulated.