Twenty-five cycles of induced ovulation with menotropins were investigated blindly with ultrasound to evaluate estrogen monitoring. Plasma 17 beta-estradiol (E2) and urinary estriol glucuronide (E3G) correlated with total ovarian volume (0.58, 0.58), total follicular volume (0.56, 0.52), volume of the largest follicle (0.53, 0.54), and days of administration of menotropins (0.49, 0.44), respectively. The mutual correlations of days of menotropin administration, volume of the largest follicle, E2, and E3G with total follicular volume explained the correlations of E2 and E3G with days of administration of menotropins and with volume of the largest follicle. Thus, multiple small follicles can reproduce the E2 or E3G levels associated with a single mature follicle if they result in the same total follicular volume. As menotropins were administered for progressively longer periods, the number of maturing follicles increased. We conclude that ultrasound appears to be useful for monitoring induction of ovulation with menotropins since it provides more accurate information on follicular number and size than can be obtained by estrogen determinations alone.