ER content of primary tumour tissue has been examined in 704 patients presenting with operable breast cancer. The median follow-up is now 84 months and no patient has received adjuvant therapy of any kind. ER status is related to histological grade, menopausal status, initial site of metastases and subsequent response to endocrine therapy. A significant advantage in terms of survival is found in ER positive patients which is confined to those lymph node positive at mastectomy. DFI is also significantly related to ER status in lymph node positive patients. Survival after the symptomatic presentation of metastases and the institution of endocrine therapy is prolonged in patients with ER positive tumours. The overall response rate to endocrine therapy in assessable patients with ER positive tumours is 32%. By combining the ER status and histological grade of tumour tissue, a group of patients comprising 28% of those assessable to endocrine therapy can be identified (ER positive, grade I and II) with a response rate of 46%.