Oestrogen receptor concentrations were measured in primary tumours of 291 postmenopausal breast cancer patients with high risk of recurrence. These patients were a subset of the 1650 patients participating in the Danish Breast Cancer Cooperative Group's trial of adjuvant treatment with tamoxifen (30 mg daily for one year). A cut-off point of 10 fmol/mg cytosol protein and the use of a Cox proportional hazards model distinguished between patients with long recurrence-free survivals and those with early recurrent disease. The use of this model also showed that patients with an oestrogen-receptor content below 100 fmol/mg did not benefit from the endocrine therapy, while those with concentrations above 100 fmol/mg had a significantly longer recurrence-free survival. This finding is consistent with the response of advanced breast cancer to endocrine treatment.