A total of 1095 patients with operable breast cancer and enrolled in a randomized clinical trial were analysed for estrogen (ER) and progesterone (PgR) receptor content of their primary tumor, and the relationships between steroid receptor status and several epidemiologic characteristics were studied. The proportion of ER+ and median ER levels increased with age: compared to women younger than 40, those aged 66 or more were approximately three times more likely to have an ER+ tumor (OR = 3.0, 95% C.I. = 1.6-5.7). This difference tended to be more marked after comparison between patients with ER greater than 100 fmol/mg protein and ER- within the same age groups: OR = 7.04, 95% C.I. = 2.89-17.12. No association emerged between age and PgR. ER status and concentrations were independent of menopausal status after adjustment for age, whereas the proportion of PgR+ and PgR levels were significantly lower in postmenopausal patients of the same age. The distribution of ER and PgR profiles was similar in relation to family history of breast cancer, reproductive events and other selected epidemiologic characteristics of the patients.