One of the most important functions of oestrogens is modulation of the immune system. By interactions with specific oestrogen receptors ERα and ERβ these sex steroid hormones are capable of regulating many aspects of both natural and adaptive immunity. Aging and the hypoestrogenic state can therefore influence immunological functions. Use of drugs belonging to SERM or isoflavones could also modify the immune response in post-menopausal women. All these phenomena might influence the risk of cancer initiation and the natural course of tumour growth.