The study investigates the relationship of follicular fluid steroids and human chorionic gonadotrophin to oocyte maturity and fertilization rates in stimulated and natural cycles. Oestradiol, progesterone, testosterone and human chorionic gonadotrophin were quantified in 129 samples of follicular fluid and the progesterone:oestradiol ratio calculated. Both stimulated cycles (short and long luteinizing hormone-releasing hormone/human menopausal gonadotrophin regimens) and natural cycles were compared. A total of 60 women were studied, 20 in each group. In the natural cycles, testosterone was significantly lower in follicles with intermediate oocytes (P = 0.015). Both oestradiol and testosterone were significantly lower in stimulated cycles compared to natural cycles (P = 0.032 and P = 0.034 respectively). In the ovarian stimulation cycles, the progesterone:oestradiol ratio was significantly higher when oocytes fertilized (P = 0.052). Moreover, in the stimulated cycles, oestradiol and human chorionic gonadotrophin were singnificantly lower in the short protocol compared to the long protocol. The data demonstrate that the hormonal milieu of the follicle is altered in down-regulated stimulated cycles to varying degrees, depending partially on the type of protocol used. Furthermore, the progesterone:oestradiol ratio, rather than individual hormone concentrations, may be a useful predictor of the fertilizing capacity of the oocytes.