Progesterone is essential in the luteal phase whereas luteal oestradiol may play only a permissive role on the endometrium. However, a rapid decline in oestradiol concentrations around the mid-luteal period may compromise the endometrial integrity leading to poor IVF outcomes. A retrospective analysis of 763 women aged <40 years undergoing their first IVF cycle and having < or =3 embryos replaced was undertaken. In cycles receiving human chorionic gonadotrophin (HCG) for luteal support, 25th, 50th and 75th centiles of the ratio of day-of-HCG oestradiol to mid-luteal oestradiol (oestradiol ratio) were 1.8, 2.8 and 5.0 respectively. Hormonal parameters were not different between pregnant and non-pregnant cycles. The outcomes were similar irrespective of the oestradiol ratio. Progesterone supplementation was used instead when the HCG oestradiol was >18 000 pmol/l or there were features of ovarian hyperstimulation syndrome. Pregnancy rates of these hyperstimulated cycles were 16.7 and 11.4% per cycle respectively when oestradiol ratio was < or =5.0 and >5.0. This difference did not reach statistical significance. Our results could not find an adverse outcome in cycles showing a rapid decline in oestradiol during the mid-luteal phase.