The determination of oestrogen receptors seems to be of prognostic value in the radically-operated breast cancer patient. Patients with negative receptor values exhibit early recurrence in a significantly higher percentage of cases than those with positive values. The prognostic implications of oestrogen receptor determination seem to be even higher than an assessment of axillary lymph node involvement. In metastatic breast carcinoma a correlation exists between response to hormonal therapy and receptor assay. The clinical implications for therapeutic management are discussed.