Isoflavones are soy phytoestrogens that have been suggested to be anticarcinogenic. Our previous study in premenopausal women suggested that the mechanisms by which isoflavones exert cancer-preventive effects may involve modulation of estrogen metabolism away from production of potentially carcinogenic metabolites [16alpha-(OH) estrone, 4-(OH) estrone, and 4-(OH) estradiol] (X. Xu et al., Cancer Epidemiol. Biomark. Prev., 7: 1101-1108, 1998). To further evaluate this hypothesis, a randomized, cross-over soy isoflavone feeding study was performed in 18 healthy postmenopausal women. The study consisted of three diet periods, each separated by a washout of approximately 3 weeks. Each diet period lasted for 93 days, during which subjects consumed their habitual diets supplemented with soy protein isolate providing 0.1 (control), 1, or 2 mg isoflavones/kg body weight/day (7.1 +/- 1.1, 65 +/- 11, or 132 +/- 22 mg/day). A 72-h urine sample was collected 3 days before the study (baseline) and days 91-93 of each diet period. Urine samples were analyzed for 10 phytoestrogens and 15 endogenous estrogens and their metabolites by a capillary gas chromatography-mass spectrometry method. Compared with the soy-free baseline and very low isoflavone control diet, consumption of 65 mg isoflavones increased the urinary 2/16alpha-(OH) estrone ratio, and consumption of 65 or 132 mg isoflavones decreased excretion of 4-(OH) estrone. When compared with baseline values, consumption of all three soy diets increased the ratio of 2/4-(OH) estrogens and decreased the ratio of genotoxic: total estrogens. These data suggest that both isoflavones and other soy constituents may exert cancer-preventive effects in postmenopausal women by altering estrogen metabolism away from genotoxic metabolites toward inactive metabolites.