Numerous women are treated with a combination of oestrogen and progestogen for contraception and hormone replacement therapy worldwide. A possible increased risk of cancer in target organs has been discussed vividly for many years. While oestrogens are clearly mitogenic for breast epithelial cells, there has been considerable uncertainty about the effects of progestogens. This article reviews current knowledge on this field, including our own data. Oestrogen receptors are down-regulated during the luteal phase, while progesterone receptors remain at a high level throughout the menstrual cycle. According to most studies, in vivo proliferation of normal breast epithelial cells is higher during the luteal phase in the vast majority of women. Normal breast tissue can convert oestrone sulphate to oestradiol. A negative correlation between the levels of circulating oestradiol and the enzyme converting oestrone into oestradiol suggests a local regulatory mechanism of tissue oestradiol formation. Serum progesterone levels correlate positively with sulphatase activity while 19-norsteroid progestogens may be inhibitory. We found that long-term continuous combined hormonal treatment with conjugated equine oestrogens and medroxyprogesterone acetate induced a proliferative response in the breasts of surgically postmenopausal macaques. The effect of combined treatment was more pronounced than that of oestrogen treatment alone. Both endogenous progesterone and exogenous progestogens increase proliferation of breast epithelial cells. Exogenous progestogens down-regulate both oestrogen and progesterone receptors. Oestrogen and progestogens may have both direct and indirect stimulating effects on proliferation. The finding of a positive correlation between insulin-like growth factor I messenger RNA and proliferation found in hormonally treated women with low receptor levels suggests the possibility of nonreceptor-mediated effects of sex steroids on proliferation, which needs to be investigated further.