The estrogen receptor (ER) assay has become a standard practice in the management of advanced breast cancer. Tumors lacking ER respond infrequently to endocrine therapy, whereas response rates of 50 to 60 percent are observed in ER+ tumors. Recent studies indicate that the ER status of the primary tumor is a good predictor of the endocrine dependence of metastatic tumors at the time of clinical relapse. Furthermore, the absence of ER in the primary tumor is an important independent prognostic indicator of higher rate of recurrence and shorter survival. Quantitative analysis of Er and an assay for progesterone receptor (PgR) are two methods for increasing the accuracy of selecting or rejecting patients for hormonal therapy; tumors with a high quantitative ER content or those with a positive PgR display the highest objective response rates. Preliminary analysis suggests that the presence of PgR may be a better marker of tumor hormone dependence than quantitative ER.