The conversion of 1β [3H] androstenedione to estrone in the presence of NADPH by breast cancer homogenates was assessed in tumors from 35 subjects together with estrogen and progesterone receptor concentration. Breast cancers from post menopausal women had significantly higher aromatase activity than those from premenopausal women. Although in many instances the local production of estrogens was sufficient to the capable of exerting a biologic effect, there was, however, no correlation with tumor estrogen or progesterone receptor content. Further studies of the biological significance of tumor aromatisation will depend on direct observation of tumor aromatase and response to suitable doses of aromatase inhibitors