Forty-five post-menopausal women with recurrent breast cancer were treated with the antioestrogen, tamoxifen, 20 mg twice daily. Clinical assessment after 12 weeks indicated that 18 (40%) showed some remission. Gonadotrophins were suppressed within two weeks to relatively constant concentrations within the post-menopausal range, responses to luteinising hormone-releasing hormone (LH-RH) did not change, and androgen concentrations remained within the normal range in all patients. Oestradiol concentrations rose steadily only in women in whom treatment failed. Serum prolactin concentrations were raised in 18 out of the 44 (41%) patients in whom they were measured; 13 of these did not respond to treatment. Treatment did not change the average prolactin concentration when this was within the normal range, but it significantly reduced prolactin concentrations in hyperprolactinaemic patients--within two weeks (P less than 0-01) in those who responded well and by six weeks (P less than 0-05) in those who showed no remission. Among patients with normal prolactin values the release of prolactin after thyrotrophin-releasing hormone was significantly greater in those with no remission than in those who responded to tamoxifen. Responses in those with hyperprolactinaemia were reduced to about half the control values, and again this change occurred faster in those who were successfully treated. Patients therefore seem to have a better chance of responding to anti-oestrogen treatment if prolactin secretion is low.