• Estrogen receptor (ER) activity was assayed in 132 patients undergoing mastectomy for primary breast cancer and in 75 patients whose initial treatment for metastatic cancer was endocrine manipulation. Estrogen receptor status correlated with response to endocrine therapy. Among patients whose ER activity was assayed in the primary tumor, ER-positive patients had an improved disease-free survival (independent of stage or nodal or menopausal status) after mastectomy when compared with ER-negative patients (P <.05). The ER-positive and ERnegative patients receiving adjuvant chemotherapy had similar disease-free survivals. These data confirm that patients with ER-positive primary tumors have a better prognosis than patients with ER-negative primary tumors. Although adjuvant chemotherapy improves the prognosis in patients with histologically positive lymph nodes in both ER-positive and ER-negative patients, the disease-free survivals in ER-negative patients may improve to a greater extent than in ER-positive patients. (Arch Surg 1982;117:753-757)