Objective To compare the effects of consumption of fermented and unfermented soy products on excretion of urinary isoflavonoid phytoestrogens and lignans in healthy men. Design A randomized, crossover trial consisting of two 9-day feeding periods following 5 days of baseline data collection. Subjects Healthy men, aged 20 to 40 years, were recruited from the University of Minnesota Twin Cities community. Of the 22 subjects who began the study, 17 completed all feeding periods. Interventions Fermented soy product (112 g tempeh) or unfermented soy (125 g soybean pieces) was consumed during each controlled feeding period. Main outcome measure Urine samples collected while subjects consumed their habitual diets and on the last 3 days of each feeding period were analyzed for isoflavonoid and lignan content by isotope dilution gas chromatography-mass spectrometry. Statistical analysis performed Comparisons of isoflavonoid and lignan excretion were analyzed using the general linear model procedure. Orthogonal contrasts were used to determine treatment differences of interest. Results Urinary excretion of isoflavonoids (equol, O-desmethylangolensin [O-DMA], daidzein, genistein) was higher and excretion of lignans (enterodiol, enterolactone) was lower when subjects consumed soy-supplemented diets than when they consumed their habitual diets (P<.05). Urinary isoflavonoid excretion and lignan excretion were similar when subjects consumed tempeh and soybean pieces diets; however, recovery of daidzein and genistein was significantly higher when subjects consumed the tempeh diet than when they consumed the soybean pieces diet (P<.002). When fed soy, 5 of 17 subjects excreted high amounts of equol. These five subjects tended to excrete less O-DMA and daidzein than the 12 subjects who excreted low amounts of equol (P<.06). Conclusions Fermentation of soy decreased the isoflavone content of the product fed but increased the urinary isoflavonoid recovery. This finding suggests that fermentation increases availability of isoflavones in soy. J Am DietAssoc. 1995; 95:545-551.