A relatively small number of cells in the normal human mammary gland express receptors for oestrogen and progesterone (ER and PR), and there is almost complete dissociation between steroid receptor expression and proliferation. Increased expression of the ER alpha (ERalpha) and loss of the inverse relationship between receptor expression and proliferation occur at the very earliest stages of tumorigenesis, implying that dysregulation of ERalpha expression contributes to breast tumour formation. There is evidence also for alterations in the ratio between the two PR isoforms in premalignant breast lesions. Elucidation of the factors mediating the effects of oestradiol and progesterone on development of the normal breast and of the mechanisms by which expression of the ERalpha and the PR isoforms is controlled could identify new targets for breast cancer prevention and improved prediction of breast cancer risk.