Tamoxifen is currently the endocrine treatment of choice for all stages of breast cancer and is the gold standard for antiestrogen treatment. Over the last 25 years, the drug has revolutionized breast cancer therapy. The extension of the use of this agent has occurred because of open dialogue between the laboratory and the clinic, in which laboratory findings led to extension of clinical use. Tamoxifen was originally discovered as part of a contraceptive research program at ICI Pharmaceuticals (now Zeneca). On the basis of the estrogen dependence of many breast cancers, tamoxifen, a potent antiestrogen, was predicted to have anticancer activity. Laboratory and animal studies demonstrated efficacy in breast cancer and an ability to block binding of estradiol to the estrogen receptor of human breast cancer. Preclinical studies showed the benefit of long-term vs short-term tamoxifen treatment, a finding duplicated in the clinic.