Isoflavones and lignans are soy phytoestrogens that have been suggested to be anticarcinogenic. The mechanisms by which they exert cancer-preventive effects may involve modulation of estrogen synthesis and metabolism. To evaluate this hypothesis, a randomized, cross-over soy isoflavone feeding study was performed in 12 healthy premenopausal women. The study consisted of three diet periods, each separated by a washout of approximately 3 weeks. Each diet period lasted for three menstrual cycles plus 9 days (averaging approximately 100 days), during which subjects consumed their habitual diets supplemented with soy protein powder providing 0.16 (control diet), 1.01, or 2.01 mg of total isoflavones per kg of body weight per day (10+/-1.1, 65+/-9.4, or 129+/-16 mg/day, respectively). A 72-h urine sample was collected during the midfollicular phase (days 7-9) of the fourth menstrual cycle in 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. Urinary excretion of isoflavonoids and lignans significantly increased with increased isoflavone consumption. Compared with the control diet, increased isoflavone consumption decreased urinary excretion of estradiol, estrone, estriol, and total estrogens, as well as excretion of the hypothesized genotoxic estrogen metabolites, 16alpha-hydroxyestrone, 4-hydroxyestrone, and 4-hydroxyestradiol. Of importance are the observations of a significant increase in the 2-hydroxyestrone/16alpha-hydroxyestrone ratio and a decrease in the genotoxic/total estrogens ratio. These data suggest that soy isoflavone consumption may exert cancer-preventive effects by decreasing estrogen synthesis and altering metabolism away from genotoxic metabolites toward inactive metabolites.