It has recently been found by various authors that despite a normal serum concentration of oestradiol (E2), the percentage of non-protein-bound or free E2 is abnormally high in breast cancer patients. Since it is the free E2 which is considered to be biologically active, confirmation of this finding would be most relevant to the pathogenesis of breast cancer. Using Hammond's centrifugal ultrafiltration dialysis method we have measured free E2 in heparinized plasma from 68 premenopausal women (a) at high familial risk of breast cancer (n = 18), (b) with benign breast disease (n = 17), (c) cured of T1N0M0 breast cancer at least 6 months previously (n = 17) and (d) normal controls matched for age, parity and Quetelet index (n = 16). Sex hormone binding globulin (SHBG) was measured as [3H]-dihydrotestosterone binding capacity. Free E2 and SHBG were also measured in the serum of (e) postmenopausal patients having breast cancer (n = 38) and (f) matched control cancer patients (n = 67). We confirmed a very good inverse correlation between log free E2 per cent and log SHBG (P less than 0.0001). The regression lines for groups (a)-(d) were not statistically different. The regression lines for groups (e) and (f) were identical and ran nearly parallel to those for groups (a)-(d) though somewhat lower. This small difference may be ascribed to menopausal status. Therefore, we found no difference in free E2 percentage, calculated free E2 concentration or SHBG between premenopausal women at risk, women with benign breast disease, patients cured for early breast cancer or having breast cancer and matched controls. However, postmenopausal breast cancer patients had a significantly higher total serum E2 concentration and, by consequence a higher calculated free E2 concentration compared to the carefully matched control group.