Seventeen premenopausal women with metastatic breast cancer were treated with the potent Luteinizing Hormone Releasing Hormone (LHRH) agonist Buserelin as a first-line agent. Twelve patients (group A) were treated with Buserelin alone and five patients (group B) with the combination of Buserelin and tamoxifen from the start of treatment. In nine patients of group A tamoxifen was added to Buserelin later on because of tumor progression or recurrent peaks of plasma estradiol (E2). Chronic intranasal therapy with Buserelin alone, preceeded by parenteral administration, caused an objective remission in four patients (2 X C.R., 2 X P.R.) and stable disease in four further patients without causing side effects. The longest duration of response until now is more than 29 months. After addition of tamoxifen a partial response occurred in two more patients of group A. Anovulation with suppressed progesterone secretion was reached in all patients treated with Buserelin alone, but transient peaks of E2 occurred in the majority (60%) of the patients. Addition of tamoxifen to Buserelin treatment caused disappearance of E2 peaks in 2 patients, but also reappearance of progesterone secretion with recurring E2 peaks in 3 other patients; in one case hyperstimulation of the ovaries was observed without progression of tumor growth. In group B only one woman showed a complete castration effect, while in four patients progesterone secretion was not (completely) suppressed. In two of these five patients an objective response occurred. In conclusion, Buserelin appears effective in the treatment of premenopausal women with metastatic breast carcinoma, but with the regimen used close control of endocrine parameters is necessary because of the variation in hormonal response with a risk of (hyper)stimulation of the ovaries, especially during combination therapy with tamoxifen.