Although sex hormone-binding globulin (SHBG) production is stimulated by estrogen, no change in SHBG has been demonstrated during the menstrual cycle. To further study possible cyclic changes in serum SHBG, 12 women with a normal menstrual and fertility history had daily SHBG measurements during a menstrual cycle. SHBG was measured by dextran-coated charcoal saturation analysis and RIA. Serum LH was measured by mouse Leydig cell bioassay and RIA, and FSH, estradiol (E2), and progesterone were determined by RIA. In 10 women, a significant increase in mean SHBG by both methods occurred during the luteal phase of the cycle, immediately after the preovulatory increase in serum E2 (P less than 0.001). Two women had no SHBG increase; although each had a significant rise in serum E2 before the LH surge, luteal phase E2 levels were similar to those in the early follicular phase. In one of these women, a rise in SHBG was demonstrated by RIA. This study demonstrates that SHBG changes during the menstrual cycle in association with E2 changes, and it appears to be a marker for the endogenous estrogen changes that occur in normal ovulating women.