Epidemiological studies revealed that foodstuffs, in particular, soy foods containing isoflavonoid phytoestrogens may reduce the risk of some hormone-dependent disease such as not only postmenopausal symptoms but also certain(breast, prostate and colon) cancers and cardiovascular disease. This review introduces the metabolism of soybean isoflavonoids by human intestinal bacteria and the binding and gene-expression activity of the metabolites towards the human estrogen receptor(hER) alpha and beta. The dietary isoflavones(daidzin and genistin) in soybean were metabolized to equol and dihydrogenistein via daidzein and genistein, respectively. The metabolites bind more strongly to hER beta than hER alpha. The binding affinity of genistein is comparable that of 17 beta-estradiol. Equol induces transcription most strongly both with hER beta and hER alpha.