Phytoestrogens are paradoxical. Because of their structural similarity to the physiological oestrogens, they have been assumed to increase the risk of breast cancer. However, nations where the largest amounts of phytoestrogens are consumed in the diet have the lowest incidence of and rate of death from breast cancer. Although these epidemiological observations do not prove that phytoestrogens have anti-cancer properties, many preclinical experiments support this concept. Some indicate that early life exposure to phytoestrogens may be critical for breast cancer prevention. Clinical studies to define the effect of phytoestrogens on breast cancer recurrence are underway. The recent discovery of a second class of oestrogen receptors, with a differential distribution among the tissues, may enable an explanation of the phytoestrogen paradox. These receptors have opened a way of utilizing phytoestrogens in the treatment of oestrogen-sensitive chronic diseases such as atherosclerosis and osteoporosis.