The aim of this study was to determine whether endogenous estradiol levels in postmenopausal women helped determine the estrogen receptor status of subsequent breast cancers. Within the Guernsey Cohort study of 6127 women, 140 have been diagnosed with breast cancer of whom 59 had estradiol assays performed and ER status available. Estradiol levels in serum and urine were measured by radioimmunoassay and ER status of tumours by immunohistochemistry. Of the individuals in the highest tertile of serum estradiol 35% had ER + ve tumours compared with 27% in the lowest tertile. In terms of 16hydroxyestrone excretion the proportions ER + ve tumours were 22% in the lowest tertile and 38% in the highest tertile. This suggests that endogenous estrogen levels do impact on the phenotype of subsequent breast cancer.