It is well-known that sex hormone-binding globulin (SHBG) concentrations in the follicle are relatively low in comparison with the corresponding estradiol (E2) levels, which are extremely high. A direct comparison of these data in stimulated and unstimulated cycles, as well as the relationship of SHBG with testosterone (T) and E2 in serum and follicular fluid (FF), is assessed.SHBG was measured in serum and FF in 42 cycles stimulated by GnRH agonists and gonadotropins and in 15 unstimulated cycles. The levels of SHBG were compared to the corresponding total estradiol and total testosterone concentrations. The analyzed FFs of 42 women, 12 of whom conceived, were randomly selected from 90 patients participating in an in vitro fertilization program. Mature oocytes were retrieved from all follicles from which FFs were analyzed.Markedly elevated SHBG was found in both the serum and the FF of stimulated cycles compared to unstimulated cycles. In contrast, serum T and E2 were significantly higher in induced than in unstimulated cycles, while there was no significant difference in FF T or E2 between the two groups of cycles. No correlation was found between serum and FF SHBG in either stimulated or unstimulated cycles. In stimulated cycles, only in FF, SHBG was significantly correlated with both E2 and T. In unstimulated cycles, no correlation was found between SHBG and either one of the corresponding steroids either in serum or in FF.The fraction of non-SHBG bound, biologically active sex steroids may be lower in the FF of stimulated than that of unstimulated cycles.