Immunocytochemical analysis was used to determine tumour oestrogen receptor (ER) content in elderly patients with primary operable breast cancer. Only those ER positive were selected for conservative treatment with tamoxifen alone. Initial response was compared with that of historical controls not selected according to ER status. Early progressive disease was markedly reduced at 6 months, from 30 per cent in the unselected control group to 2 per cent in the study group (P < 0.001). Immunocytochemical analysis is useful for the initial selection of elderly patients with breast cancer who may be treated with tamoxifen alone.