Comparisons between oestrogen-receptor (RE)-positive or negative patients were made on a continuous series of 735 patients with primary breast tumours seen at the major treatment centre in British Columbia between 1975 and 1978. RE positivity was commoner in older patients, and was not associated with menopausal status independently of age. The concentration of receptor protein also increased with increasing age, but was not affected by menopausal status. Neither RE status nor quantity was associated with any of the epidemiological risk factors studied, which included parity, age at first birth, weight, family history and exposure to oestrogenic drugs and oral contraceptives. Patients with RE- tumours were more likely to present with symptoms other than a breast lump, pain or nipple inversion, and had less-differentiated tumours; they did not differ from RE+ patients in terms of stage, size of tumour, or interval from first symptom. These findings are discussed in terms of the biological origin and determinants of oestrogen receptors.