Isoflavone intake is associated with various properties beneficial to human health whichare related to their antioxidant activity, for example, to their ability to increase LDLoxidation resistance. However, the distribution of isoflavones among plasma lipoproteinshas not yet been elucidated in vivo. Therefore, the objective of thepresent study was to investigate the association between daidzein (DAI) and lipoproteinsin human plasma upon administration of the aglycone and glucoside form. Five men aged22–30 years participated in a randomised, double-blind study in cross-overdesign. After ingestion of DAI anddaidzein-7-O-β-d-glucoside (DG) (1 mg DAIaglycone equivalents/kg body weight) blood samples were drawn before isoflavoneadministration as well as 1, 2, 3, 4·5, 6, 8, 10, 12, 24 and 48 hpost-dose. Concentrations of DAI in the different lipoprotein fractions (chylomicrons,VLDL, LDL, HDL) and in the non-lipoprotein fraction were analysed using isotope dilutioncapillary GC/MS. The lipoprotein fraction profiles were similar for all subjects andresembled those obtained for plasma in our previously published study. The lipoproteindistribution based on the area under the concentration–time profiles from0 h to infinity in the different fractions were irrespective of the administeredform: non-lipoprotein fraction (53 %) > LDL (20 %) > HDL(14 %) > VLDL (9·5 %) > chylomicrons(2·5 %). Of DAI present in plasma, 47 % was associated tolipoproteins. Concentrations in the different lipoprotein fractions as well as in thenon-lipoprotein fraction were always higher after the ingestion of DG than of DAI. Takentogether, these results demonstrate an association between isoflavones and plasmalipoproteins in vivo.