A quantitative method based on ion-exchange chromatography and isotope dilution gas chromatography-mass spectrometry using deuterated internal standards for the determination of plasma lignans and isoflavonoid phytoestrogens has been developed. These compounds are potential anticancer compounds originating from the diet. The method allows the assay of seven different compounds, while unconjugated (free) and sulfated compounds are determined separately from glucuronides and sulfoglucuronides. Results of plasma analyses in Finnish women and in Japanese men are presented. Total concentrations of individual compounds vary greatly between the subjects (from pmol/l to mumol/l), with the Finnish vegetarians and Japanese subjects having higher values. The Finns have relatively high lignan and the Japanese very high isoflavonoid values. It is concluded that the new method allows the determination of the whole plasma lignan and isoflavonoid phytoestrogen profile.