Various pharmacological activities of phytoestrogens have become apparent. High levels of isoflavonoids among Japanese are associated with a low incidence of hormone-dependent cancers, but nothing is known about transfer through the placenta of these compounds. We studied seven young healthy voluntary Japanese women at delivery measuring six phytoestrogen metabolites in maternal and cord plasma and in amniotic fluid by isotope dilution gas chromatography-mass spectrometry. The women consumed their habitual normal Japanese soy-containing diet. Total maternal plasma isoflavonoid concentrations ranged from 19 to 744 (mean, 232 nmol/liter), cord plasma values from 58 to 831 (mean, 299 nmol/liter), and amniotic fluid values from 52 to 779 (mean, 223 nmol/liter). Total maternal plasma lignan values ranged from 1 to 85 (mean, 14.6 nmol/liter), cord plasma values ranged from 0.1 to 16 (mean, 3.5 nmol/liter), and amniotic fluid values ranged from 0.1 to 61 (mean, 15.1 nmol/liter). Phytoestrogens were detected in cord blood and amniotic fluid both in the free + sulfate fraction as well as in the form of glucuronides. The high levels of isoflavonoid phytoestrogens found in normal newborn Japanese infants indicate transfer of isoflavonoids from the maternal to the fetal compartment and suggest that soy intake in normal amounts does not involve any risk for the fetus and the newborn. It is speculated that these compounds may modify estrogen metabolism and action during fetal life and perhaps influence breast cancer risk in adulthood.