A group of anovulatory patients with polycystic ovaries (PCO) was given clomiphene citrate and compared with three control groups: normal women having spontaneous, ovulatory cycles, patients with PCO having spontaneous, regular, ovulatory cycles, and anovulatory patients without PCO. Comparisons were made at precise points of the menstrual cycle (taking the day of ovulation as day 0), using ultrasound estimates of mean follicular diameter, uterine volume, and endometrial thickness, and biochemical measurements of LH, FSH, oestradiol (E2), testosterone (T), progesterone (P) and sex-hormone-binding globulin (SHBG). Before clomiphene treatment, the anovulatory patients with PCO had significantly lower levels of SHBG and higher follicular phase concentrations of LH than all three control groups. After two cycles of clomiphene-induced ovulation, the serum LH concentration fell significantly and levels of SHBG increased significantly to levels similar to those found in spontaneously ovulating women with normal ovaries. It is likely that the loss of the usual considerable rise in E2 in both the follicular and luteal phases of ovulatory cycles is the main reason for the low SHBG found in the PCO syndrome. The loss of the normal P-induced gonadotrophin suppression may be a factor in allowing LH levels to rise.