Endometrial and follicular development were investigated by ultrasound in 25 normally cycling women who received clomiphene citrate and human menopausal gonadotrophin to induce multiple follicular development for in vitro fertilization. Ultrasonic determination of endometrial thickness and reflectivity grading and follicular number were correlated with daily concentrations of estradiol (E2) and progesterone (p) in peripheral serum. Serum E2 showed a better positive correlation with endometrial thickness than with total number of developing follicles. There was a significant inverse correlation between endometrial thickness and plasma p concentration. The E2 value per follicle and E2/p ratio were both weakly correlated respectively with endometrial thickness. These data indicate that ultrasound determination of endometrial thickness is a useful ultrasonic parameter for monitoring ovarian function for in vitro fertilization. However, endometrial thickness should always be combined with total number of developing follicles, in order to reach decisions concerning timing of oocyte recovery.