In 23 follicular fluids (FF) each yielding an oocyte known to result in a clinical pregnancy after in vitro fertilization, the following substances were measured: free and total concentrations of estradiol (E2), progesterone (P4), testosterone (T), androstenedione (A), immunoreactive inhibin, insulin-like growth factor-binding protein-1, alpha 1-antitrypsin, and placenta protein-14. In addition, FF volume and follicular diameter at the time of oocyte recovery were recorded. The characteristics of these pregnancy-associated follicles were compared with those of FF obtained from women who failed to conceive after embryo transfer. The FF were collected from 14 women who became pregnant and 14 women who did not conceive. The ultrashort GnRH agonist protocol was used for ovarian stimulation. Pregnancy was associated with follicles showing a significantly higher E2/T ratio than follicles in which the oocyte failed to implant or did not cleave in vitro (P < 0.01). In addition, FF volume and follicular diameter were significantly higher in pregnancy-associated follicles than in follicles in which the oocyte failed to implant or did not cleave in vitro (P < 0.05). The E2/A ratio was higher in pregnancy-associated follicles than in follicles in which the oocyte failed to implant, but the difference did not reach significance (0.05 < P < 0.1). No difference was found when pregnancy-associated follicles and follicles not associated with pregnancy were compared with respect to the levels of free and total E2, P4, T, A, immunoreactive inhibin, insulin-like growth factor-binding protein-1, alpha 1-antitrypsin, placental protein-14, and the E2/P4 ratio. This study demonstrates a correlation between the pregnancy potential of oocytes and the ratio of E2 to androgens in FF. It confirms and extends the correlation between the E2/androgen ratio and follicular health and maturity. A low E2/androgen ratio seems to express early follicular atresia, which affects the viability of the enclosed oocyte negatively and limits the chances of a resulting preembryo to implant and establish itself as a pregnancy.