Phyto-oestrogens have been suggested to have a preventive effect against various cancers. This review includes a discussion of the consumption of phyto-oestrogen-rich foods such as soy, a source of isoflavones, and whole grain products, which contain lignans, and their role in the prevention of breast, prostate, and colon cancer. In women, a soy-containing diet is only slightly protective against breast cancer, if at all, but is more likely to be beneficial if initiated before puberty or during adolescence. These findings are supported by conclusions of studies of immigrants and other epidemiological studies. However, in one case-control study and one prospective study, a low-lignan diet increased the risk of breast cancer. Experimental evidence also exists for an inhibitory effect of soy and rye bran on prostate-cancer growth and for rye bran or isolated lignans on colon cancer. Whether these observed protective effects are caused by the presence of dietary phyto-oestrogens, or whether they are merely indicators of a healthy diet in general, has not been established.