Ovulation induction with human gonadotropins (hMG or pFSH and hCG) was simultaneously monitored with daily serum 17 beta-estradiol measurements and ultrasound in 38 infertile women. They were subgrouped as follows: A, hypogonadotropic hypogonadism (n. 7); B, polycystic ovary syndrome (n. 13); C, non-PCO chronic anovulation (n. 6); D, unexplained infertility (n. 12). The dose of hMG and the duration of therapy were assessed individually depending on the results of the monitoring when serum estradiol concentration was 500-1500 pg/ml and/or ultrasound showed of a follicle of at least 18 mm, hMG was stopped and hCG was administered 36 hours later. Ovulation was obtained in 61 cases and pregnancy occurred in 12 women out of 64 monitored cycles. The highest pregnancy rate was observed in hypogonadotropic hypogonadism; this group also required the greatest total dose of hMG to obtain ovulation. We confirm that women with PCOS are at increased risk with this therapy, since 5 subjects in our series showed signs of ovarian hyperstimulation in spite of the lowest total dose of FSH administered to this group. We conclude that anovulatory states of various origin are associated with different ovarian sensitivity.