Consumption of soy protein has been associated with altered risk of developing endocrine-regulated cancers. This study was designed to assess the independent effect of soy relative to animal protein and soy-derived isoflavones on circulating estrogen and androgen concentrations in postmenopausal women and older men. Forty-two subjects (> 50 yr) with low-density lipoprotein cholesterol levels of > or = 3.36 mmol/l were fed each of 4 diets in randomized order for 6 wk/phase. All food and drink were provided. Diets contained 25 g soy or common sources of animal protein/4.2 MJ containing trace or 50 mg isoflavones/4.2 MJ. At the end of each diet phase, concentrations of estrone sulfate, estrone, estradiol, testosterone, androstendione, dihydrotestosterone, dehydroepiandrosterone, and dehydroepiandrosterone sulfate were measured. In postmenopausal women, concentrations of estrone were higher and its precursor, dehydroepiandrosterone, lower after consuming the soy compared with animal protein diets (P = 0.0396 and 0.0374, respectively). There was no significant effect of isoflavones on any of the hormones measured. In older men, dehydroepiandrosterone sulfate concentrations were lower after consuming the isoflavone (P = 0.0106) and higher after soy, compared with the animal protein diets (P = 0.0118). These data suggest that relatively large amounts of soy protein or soy-derived isoflavones had modest and limited sex-specific effects on circulating hormone levels.