Approximately 30% of all breast cancer patients will respond to some form of endocrine therapy. The rate of response depends on the level of estrogen receptors (ER) and progesterone receptors (PgR) in the tumor tissue. While approximately 60% of ER-positive advanced breast tumors will respond objectively to endocrine therapy, up to 80% of both ER-positive and PgR-positive tumors will respond. The quantitative assessment of receptors in tumor specimens has also led to the use of ER and PgR as prognostic indicators of disease-free intervals following mastectomy. In general, patients with steroid hormone receptor-positive primary tumors have longer disease-free intervals before recurrence than those with hormone receptor-negative tumors. Receptor levels are related to age: lower levels of ER are observed in younger women compared with older women. The recent development of monoclonal antibody assays for ER may allow for more precise predictions of the response of patients to hormonal manipulation.