Continuous administration of gonadotrophin-releasing hormone (GnRH-)analogues leads to a receptor-down regulation of pituitary GnRH-receptors and subsequently inhibits ovarian hormone production. Since October, 1984, 118 evaluable pre- and perimenopausal patients (median age 42, range 25-55 years) with metastatic breast cancer were entered into an open phase II multicenter trial to evaluate efficacy of this new treatment modality. Patients were treated with the GnRH-analogue Goserelin (3.6 mg depot s.c. every 4 weeks) as first line therapy and followed up until progression. Mean serum gonadotrophins LH and FSH were significantly suppressed by Goserelin. Within 2-3 weeks, mean serum E2 values decreased to values seen in castrated women (less than 30 pg/ml). Overall objective response with complete and partial remissions (CR + PR) was achieved in 44.9% of patients with a median time to progression (mTTP) of 59 weeks, (range 20-163 weeks), no change (NC) in 28.0% with a mTPP of 27 weeks (range 16-101 weeks), and progression (P) in 27.1%. Responses were seen in ER-positive as well as ER-negative tumors, and in patients with different sites of metastases (locoregional, bone, visceral, multiple). The value of different prognostic factors in relation to response rates, time to progression and time to death (overall survival) is discussed. Median overall survival (time from beginning of palliative Goserelin treatment to death) was 148 weeks.(ABSTRACT TRUNCATED AT 250 WORDS)