The aim of this study was to elucidate the roles of free estradiol and free progesterone in endometrial development and the outcome of IVF-ET. Ovarian hyperstimulation with long GnRHa-hMG protocol was performed for IVF-ET. Proliferation and maturation in endometrium was evaluated by transvaginal sonography. Total steroid hormone levels in serum were determined by EIA and free steroid hormone levels were determined by ultrafiltration dialysis. Total estradiol and total progesterone levels were significantly higher in conception cycles than that in non-conception cycles after Day 14 (Day 0 is the day of hCG administration). In the case of free estradiol and free progesterone, the difference was seen as early as Day 11. The hyperechoic endometrial area ratios, which represent endometrial maturation, evaluated by sonography on Day 2 in conception cycles (75%) were higher than those in non-conception cycles (56%), although there was no difference in total estradiol and total progesterone levels on Day -2, 1, 2, but the percentage of free progesterone in conception cycles (2.5%) was higher after Day 1 than that in non-conception cycles (2.0%). These results suggest that the percentage of free progesterone played an important role in successful conception and could be an indicator to estimate conception.