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Estradiol (E2) circulates in the blood in three states: unbound (U-E2), bound to sex-hormone binding globulin (SHBG-E2), and bound to albumin. There is evidence to support the concept that only U-E2 and albumin-bound E2, are bioavailable (i.e., rapidly extracted by tissues). A case-control study nested within a large cohort of women, in which we are examining the effect of estrogens on breast cancer risk, offered the opportunity to assess the reliability of measurements of E2, the percentage of SHBG-E2, and the percentage of U-E2 based on multiple annual serum specimens. Long-term (1-2 year) reliability, as estimated by the intraclass correlation coefficient, was assessed in a subgroup of 71 premenopausal and 77 postmenopausal controls for whom two or three serum specimens were assayed. In postmenopausal women the intraclass correlation coefficient for a single measurement of total E2 was only 0.51. As for the percentage of SHBG-E2, intraclass correlation coefficients were 0.83 and 0.94, and for U-E2, 0.72 and 0.77 in the premenopausal and postmenopausal groups, respectively. These data suggest that, whereas single determinations of total E2 are insufficient to reliably estimate a woman's true mean level, a single measurement of the percentage of SHBG-E2 or U-E2 is adequate to assess bioavailability of E2 in an epidemiological study, irrespective of day of the menstrual cycle.