Estrogen receptor protein (ERP) determinations of primary cancers of 1034 patients with primary breast cancer were done. ERP-positive patients tended to have a lower recurrence rate and had significantly improved survival. This difference was most apparent in patients with four or more axillary nodes involved. ERP-positive patients who recurred had a better survival. ERP did not influence response that adjuvant chemotherapy, nor did the presence of progesterone receptor or femtomole level of ERP affect recurrence.