Twelve premenopausal patients with advanced breast cancer were entered into pilot phase II study to assess efficacy, toxicity and influence of the synthetic LH-RH agonistic analogue D-Trp6-LH-RH (Decapeptyl®) on the patients’ hormonal status. The patients, aged 33-50, with newly diagnosed stage IV or recurrent breast cancer were not previously treated by any kind of endocrine therapy. Steroid receptor status was known in 9 patients. Decapeptyl was applied monthly at a dose of 3.75 mg i.m. until progression. The therapeutic response was evaluated in 11/12 patients. Partial remission was achieved in 5, stabilization in 3, and 3 consecutive patients failed to respond. The best therapeutic response was obtained in patients with pleuropulmonal and soft-tissue involvement, aged 41-45, including those with incomplete ovarian suppression, and regardless of steroid receptor status. The mean serum gonadotropins and estradiol levels were suppressed. The treatment was free of any side effects, except hot flushes in 7 patien