Hormones and hormone level modifying substances have long been used to treat hormone-dependent tumours in humans. Recently, attempts have been made to use hormone manipulation regimens for the treatment of these tumours in veterinary medicine. The aim of the present study was to evaluate the activity of the luteinizing hormone-releasing hormone (LHRH)-agonist, D-ser(But[t])[6]-Azgly[10]-LHRH (Goserelin) in hormone-dependent mammary cancer in dogs. Eighteen female dogs with hormone-dependent mammary cancer (T2-T4, N0, M0 according to TNM clinical staging classification) were selected and allocated into two groups: nine dogs not treated with Goserelin (Group 1) referred to as control; and nine dogs treated with 60 microg/kg depot Goserelin every 21 days for 12 months (Group 2). Goserelin treatment decreased circulating levels of oestradiol and progesterone and reduced the size of mammary tumours; all the animals showed objective response (OR) to treatment after 3 months, and the relapse-free survival after 2 years was 88%. Haematology and blood chemistry parameters, measured every month from the beginning of treatment, as well as physical examination, showed that the drug was without toxic effects. This suggests that, at the dose administered, Goserelin blocks the hypothalamus-pituitary-ovary axis, and consequently can be useful to treat hormone-dependent mammary tumours in female dogs.