Abstract Quantitative values of oestradiol receptor were determined in the primary breast cancer of 526 patients, 106 of whom have so far required hormonal therapy for metastatic disease (premenopausal patients—oophorectomy; postmenopausal patients—tamoxifen 20 mg twice daily). The rate of response to treatment was significantly higher in patients with receptor-positive primary cancers, and the likelihood of response further increased in proportion to the measured receptor concentration. Preliminary data also suggest that patients with high receptor values in the primary tumour will enjoy a longer remission than those with low measured values. Assays of oestradiol receptor were also carried out on biopsy specimens of accessible metastases, largely skin and lymph-node recurrences, in 24 patients who were treated mainly with local measures. Receptor status was the same in both the primary and the secondary tumour in only 18 patients.