Premenopausal Study. Twenty-five pre- or perimenopausal patients with advanced breast cancer were treated with leuprorelin acetate 3.75 mg (n = 9) or 7.5 mg (n = 16) every 4 weeks. Serum levels of gonadotrophins and oestrogens were suppressed markedly by both doses and there was no indication that the lower dose was less effective as an oestrogen suppressant. There were four objective responders to the 3.75 mg dose and six to the 7.5 mg dose. Toxicity was confined almost entirely to hot flushes, which occurred in 17 patients. Leuprorelin acetate is therefore an effective agent in the treatment of premenopausal breast cancer patients. There appears to be no major detriment to the use of 3.75 mg rather than the 7.5 mg dose. Postmenopausal Study. Fifteen postmenopausal patients with advanced breast cancer were treated with monthly injections of leuprorelin acetate 7.5 mg to assess the clinical activity and endocrine responses to treatment. None of the 15 patients showed an objective response to treatment, although four patients had stable disease for at least 6 months. Endocrine effects after 4 weeks' treatment included major suppression of serum gonadotrophins to below 10% of pretreatment values and decreases in the level of serum testosterone in 12 of 14 patients. In this group there were no changes in oestradiol levels, although we had previously observed suppression in postmenopausal patients treated with goserelin. In common with other gonadotrophin-releasing hormone analogues, leuprorelin acetate cannot be recommended as a treatment for postmenopausal breast cancer.