We have studied the relationship between mammographic parenchymal pattern of the breast and oestrogen receptor (ER) status in the primary tumour in 337 patients with operable invasive breast cancer, and the response to endocrine therapy in patients subsequently developing secondary disease. Patients with dysplastic (DY) parenchymal pattern are significantly more likely to develop tumours which are ER-positive. In 92 patients who subsequently developed secondary disease, those whose primary tumour arose in a DY pattern breast were significantly more likely to respond to endocrine therapy irrespective of their ER status.