Abstract Plasma concentrations of prolactin, FSH, LH and oestradiol- 17β were measured in patients with advanced breast cancer being treated with tamoxifen. Plasma FSH levels dropped during the first month of treatment in patients who subsequently responded to therapy and in those who did not. After prolonged therapy plasma FSH in the responders returned to pre-treatment levels whereas those in the non-responders remained lower than the pre-treatment value. LH levels were lowered during tamoxifen therapy but there was no significant difference between the two group of patients. Plasma prolactin and oestradiol- 17β concentrations were unaffected by treatment.