Estrogen receptor protein is known to be an important prognostic factor for patients with breast cancer. The presence of estrogen receptor correlates with response to endocrine therapy in patients with metastatic disease and is associated with prolonged disease-free and overall survival in patients with primary disease. But the correlation between estrogen receptor positivity and endocrine dependence is not perfect. Approximately 40% of estrogen receptor positive tumors fail to regress with endocrine therapy. It has been hypothesized that another protein, progesterone receptor, may be a more effective marker of endocrine responsiveness since progesterone receptor is the end product of estrogen action. We have examined the relationship between progesterone receptor and response of advanced breast cancer tumors to hormonal manipulations. Promising retrospective results indicate the need for new, prospective clinical trials to further define the prognostic value of progesterone receptor for these tumors. We have also analyzed the disease-free intervals of patients with primary disease and found that progesterone receptor was more important than estrogen receptor for predicting time to recurrence. We suggest that both estrogen receptor and progesterone receptor be routinely measured in all breast cancer tumors, and that the results of these assays will help the physician individualize therapy for breast cancer patients.