Daily changes in the thickness and texture of the endometrium were assessed by ultrasonography in 15 natural ovulatory cycles. The endometrium in the follicular phase became thicker every day until ovulation [5.8 +/- 1.8 mm (mean +/- SD) on day -8, 10.7 +/- 2.9 mm on day 0, day 0: ovulation], but the thickness did not change during the luteal phase. There was a significant correlation between the thickness of the endometrium and the serum estradiol concentration in the follicular phase (r = 0.64, n = 89, p less than 0.01), but the change in the thickness of the endometrium varied with the individual. The texture of the endometrium changed in the menstrual cycle. The hyperechoic area of the endometrium spread from the basal layer to the lumen during the early-mid luteal phase. The ratio of the thickness of the hyperechoic area to the total thickness of the endometrium (hyperechoic endometrial area ratio, HEA ratio) increased from 34.3 +/- 11.0% on day 0 to 100% on day 9. There was a significant correlation between the HEA ratio and the serum progesterone concentration in the period from day -2 to day 9 (r = 0.68, n = 91, p less than 0.01).