Plasma levels of luteinizing hormone, sex hormone binding globulin, and antithrombin III were measured in pre- and postmenopausal breast cancer patients receiving adjuvant combination chemotherapy or combination chemotherapy and long-term tamoxifen therapy. The aim was to determine the estrogen-like effects of tamoxifen. The premenopausal patients had received tamoxifen for between 434 and 2592 days and postmenopausal patients between 91 and 1560 days. Tamoxifen caused a consistent rise in sex hormone binding globulin in premenopausal (P less than 0.03) and postmenopausal (P less than 0.01) patients compared to chemotherapy controls. Luteinizing hormone levels were only significantly lowered (P less than 0.008) in premenopausal patients compared to chemotherapy controls. Antithrombin III levels were significantly depressed (P less than 0.001) in postmenopausal patients compared with chemotherapy controls. However, none of the patients had a value that was depressed by more than 30% of the laboratory control (the level of clinical significance). The mean for the group was 90% +/- 4 (mean +/- SD, N = 11). The estrogen-like rise in sex hormone binding globulin produced by adjuvant tamoxifen therapy could be advantageous for maintaining the antiestrogenic action of the drug. A decrease in antithrombin III occurred but was not within a range of clinical concern. We recommend, however, that patients with a history of thromboembolic disorders should be monitored carefully if placed on tamoxifen therapy.