In 42 postmenopausal women with breast cancer aged 48-85 (mean age 62.4) years, the blood sex hormone levels were measured before and after 6 months of tamoxifen administration (20 mg daily). Follicle-stimulating hormone and luteinizing hormone levels decreased after tamoxifen administration (p < 0.001), but remained in the postmenopausal range, oestradiol levels increased (p < 0.05), sex hormone binding globulin levels increased (p < 0.001), testosterone levels remained stable (p > 0.1), free testosterone levels decreased (p < 0.001), delta4-androstenedione, 17-hydroxyprogesterone, and dehydroepiandrosterone sulfate levels remained unchanged (p > 0.1), and basal prolactin levels and their response to thyrotrophin-releasing hormone injection decreased significantly (p < 0.001) after tamoxifen therapy. It is concluded that tamoxifen has many and diverse effects on sex hormone levels, and its adverse effects do not affect the biological status of the patient, except perhaps for oestradiol, that increases in some cases, whose possible effect must be studied.