In polycystic ovary syndrome (PCOS), follicle development arrests in the early antral stage when aromatase expression in the granulosa cells (GC) would normally occur. Despite high intrafollicular concentrations of androstenedione and bioactive FSH, in vivo estrogen biosynthesis remains low. When GC from PCOS follicles are stimulated with FSH in vitro, a marked stimulation of estrogen production occurs, suggesting that PCOS follicles contain an endogenous inhibitor of estrogen production. To test this hypothesis, GC from hyperstimulated women were cultured with increasing concentrations of follicular fluid (FF) from PCOS and normally cycling control women in the presence of androstenedione (10(-5) mol/L). FF from control women caused a small decrease (20%) in estradiol production. PCOS FF caused a dose-related inhibition of estradiol production (60%), indicating that there was significantly more inhibitory activity in PCOS FF. To determine whether abnormal androgen metabolism could play a role in inhibiting estradiol production in PCOS, we measured 5 alpha-androstane-3, 17-dione, a competitive inhibitor of aromatase activity, in serum and FF of control and PCOS women. 5 alpha-Androstane-3, 17-dione levels in serum were significantly elevated in PCOS. 5 alpha-Androstane-3, 17-dione levels were 1000-fold higher in PCOS FF than serum. Moreover, FF levels were markedly higher in PCOS follicles (P < 0.0001) than in normal dominant and cohort follicles. Dose-response studies revealed that the concentration of 5 alpha-androstane-3, 17-dione present in FF form normal dominant follicles (79.4 +/- 14.6 nmol/L) had little effect on estradiol production. In contrast, 5 alpha-androstane-3, 17-dione levels in PCOS FF (581.6 +/- 62.9 nmol/L) inhibited estradiol production by 75%. These data support the hypothesis that PCOS FF contains one or more endogenous inhibitors of aromatase activity and suggest that abnormally high 5 alpha-androstane-3, 17-dione levels in PCOS FF may be an important inhibitor of estradiol production.