ICI 182,780 (Fulvestrant) is the first in a new class of novel, steroidal, 'pure' antioestrogens--the oestrogen receptor (ER) down-regulators. Its unique mode of action and the absence of partial agonist activity make it a candidate for the treatment of advanced breast cancer in both pre- and postmenopausal women. Tamoxifen has been available for use over the past 25 years. However, its partial agonist activity has been associated with detrimental effects, particularly on the endometrium, and may be associated with the development of tamoxifen resistance. Other antioestrogen agents have previously been unable to demonstrate clinically relevant activity following the development of resistance to tamoxifen. In contrast, the unique mechanism of action of ICI 182,780 results in significant clinical activity in patients failing on tamoxifen therapy. Indeed, phase III clinical trials have demonstrated that ICI 182,780 is at least as effective as the aromatase inhibitor anastrozole in the treatment of postmenopausal patients with advanced disease who have progressed during threatment with prior enocrine therapy. As such, ICI 182,780 will provide a valuable addition to the armamentarium for the treatment of advanced breast cancer.