Tamoxifen 1 is a triphenylethlene oestrogen antagonist which has partial oestrogen agonist activity in some species. Most therapeutic trials with tamoxifen have involved postmenopausal women with advanced brease cancer, about 25 to 60% of patients showing same improvement while receiving treatment (usually 20 or 40mg daily); however- poorly defined assessment criteria in some studies make it difficult to compare the results of different authors. 7 to 18% of patients have had complete clinical remissions lasting for a few months to several years. Although few comparative studies with other treatments have been done, it nevertheless appears that tamoxifen is at least as effective as standard oestrogen or androgen therapy in postmenopausal women, while producing a lower incidence of troublesome adverse effects. In the only comparison with a cytotoxic treatment regimen, the response rates with tamoxifen and cytotoxic therapy were similar (41% versus 50%) in patients who were more than 5 years postmenopausal and had primarily soft-tissue involvement; but in patients who were within 2 to 5 years of their last menstrual period cytotoxic treatment was more effective. Similarly, most authors have reported a higher response rate with tamoxifen in women several years postmenopausal than in those more recently postmenopausal. Tumours which contain receptors respond more often to tamoxifen than those which do not. Administering tamoxifen concomitantly with other chemotherapeutic agents such as cytotoxic drugs appears to increase the response rate, but as might be expected also increases the incidence of adverse reactions. When used alone tamoxifen has been relatively well tolerated in all studies, the overall withdrawal rate due to side-effects being less than 3%.