Plasma sex hormone-binding globulin (SHBG), testosterone, total oestradiol (E2) and proportion of E2 not bound to blood proteins (percentage free E2) have been measured in a case-control study to test whether any of these factors are related to the risk of breast cancer. The cases studied were 27 premenopausal and 34 postmenopausal women with newly diagnosed localized breast cancer who were admitted to the All-Union Cancer Research Centre, Moscow. The controls were an equivalent number of women from the same neighbourhood. These were matched for age (within 2 years) and menopausal status. The average levels of E2 in cases were significantly higher than controls (p = 0.004 and p < 0.001 for pre- and postmenopausal groups, respectively). Percentage free E2 was significantly raised only in premenopausal cases (p = 0.019). The plasma concentrations of testosterone and SHBG were similar for both pre- and postmenopausal groups. Raised E2 or percentage free E2 was associated with an increased relative risk of breast cancer. However, only E2 levels (crude or adjusted for body build) were significantly related to risk in the two menopausal sets (p = 0.022 and p = 0.002, respectively). High percentage free E2 was associated with almost a threefold increase in the risk of breast cancer in premenopausal women; however, this increase was not statistically significant (p = 0.083). The levels of SHBG or testosterone, crude or adjusted for body build and E2 concentration, were not related to risk of breast cancer.