In estrogen target tissues and hormone-dependent tumors, the steroid enters the cells and binds to a cytoplasmic protein called the estrogen receptor (ER). The steroid-receptor complex then migrates to the nuclei, where it initiates the biochemicial events characteristic of estrogen stimulation. Since ER is absent in tissues not responsive to estrogen, recent studies have asked whether ER assays in human breast cancer tissue might be used to identify those patients likely to respond to endocrine therapy. Data on 436 clinical trials contributed from a dozen centers around the world now clearly indicate that if a patient's tumor does not contain ER, there is virtually no chance of tumor regression following endocrine therapy. A large number of patients can be thus spared unrewarding major endocrine ablative therapy if ER assays are performed routinely. Of tumors with positiev ER, 55-60% respond to endocrine therapy. This single piece of data, when coupled with available clinical prognostic factors such as menopausal status, disease free interval, site of the dominant lesion, and especially response to previous hormonal therapies, should be practicing oncologist to select or reject endocrine therapy with considerable confidence.