To examine the value of the measurement of oestradiol/progesterone ratios in the peri-implantation period, three different types of cycle were studied; natural cycles (n = 13), cycles treated with anti-oestrogens (n = 19), and artificial cycles (n = 41) produced by six different hormone replacement regimens. Normal endometrial development was found to occur across a wide range of oestradiol/progesterone ratios. There was no significant correlation between the oestradiol/progesterone ratios and results of endometrial dating. The results suggest that oestradiol/progesterone ratios are of limited value in predicting endometrial development at the time of implantation.