We have previously observed that exogenous oestradiol can delay the intercycle increase in plasma follicle stimulating hormone (FSH). The increase in plasma FSH that follows discontinuation of exogenous oestradiol peaks after 3 days. We have now studied the possibility of using exogenous oestradiol to synchronize the increase in endogenous FSH with the onset of human menopausal gonadotrophin (HMG) treatment in controlled ovarian hyperstimulation (COH). A total of 30 women aged 35.1+/-6.3 years (mean+/-SD) undergoing ovarian stimulation received 2 mg of oestradiol valerate twice daily starting on day 25 of the previous menstrual cycle until the first Tuesday following menses. Ovarian stimulation was initiated 3 days later. On the last day of oestradiol treatment, plasma oestradiol, FSH and luteinizing hormone (LH) (mean+/-SEM) were 566+/-53 (pmol/l), 3.8+/-0.4 (IU/l) and 5.5+/-0.8 (IU/l) respectively. After 3 days, the FSH and LH (mean+/-SEM) had increased to 6.7+/-0.7 and 6.9+/-0.7 (IU/l) respectively while oestradiol decreased to 251+/-29 (pmol/l). The mean number (+/-SEM) of HMG ampoules used was 25.1+/-2.7 and treatment lasted 11.3+/-0.9 days. Five women became pregnant for a pregnancy rate (ongoing) of 19 (15)%. If all women aged >40 years (six women who did not become pregnant) were excluded from analysis the pregnancy rate (ongoing) was 24 (19%). These results indicate that exogenous oestradiol can safely be used for the synchronization of endogenous and exogenous FSH stimuli in COH. This approach provides the practical advantage of permitting an advanced timing of the onset of COH treatments when gonadotrophin-releasing hormone (GnRH) agonists are not used, which improves treatment convenience for patients and team members alike. Further development of this model may enable control of the onset of natural cycles which may find practical applications for timing assisted reproductive techniques (intrauterine insemination or in-vitro fertilization) in the natural cycle.