Lignans and isoflavonoids are diphenolic compounds found in plant foods, particularly whole grains and legumes. They have been shown to have anticarcinogenic properties in animal and cell studies, and have been associated with reduced cancer risk in epidemiological studies. In order to perform further epidemiological and metabolic studies on these compounds, it is necessary to be able to monitor concentrations in biological samples. In this study, we examined the effects of consumption of flaxseed, a concentrated source of lignans, on fecal lignan excretion and evaluated the effect of high lignan consumption on fecal excretion of isoflavonoids. Thirteen women were studied for two diet periods of three menstrual cycles each in a cross-over design. During the control period, they consumed their usual diets; during the treatment period they consumed their usual diets supplemented with 10 g/day ground flaxseed. Feces were collected on days 7-11 of the last menstrual cycle in each diet period. Five-day fecal composites were analyzed for lignans and isoflavonoids by isotope dilution gas chromatography-mass spectrometry. Fecal excretion of the lignans enterodiol, enterolactone, and matairesinol increased significantly with flax consumption, from 80.0 +/- 80.0 (SD) to 2560 +/- 3100; 640 +/- 480 to 10,300 +/- 7580; and 7.33 +/- 10.0 to 11.9 +/- 8.06 nmol/day, respectively. There were no differences in fecal excretion of the isoflavonoids, daidzein, equol, genistein, and O-demethylangolensin.