Soybean consumption is associated with reduced rates of breast, prostate, and colon cancer, which is possibly related to the presence of isoflavones that are weakly estrogenic and anticarcinogenic. We examined the effects of soya consumption on circulating steroid hormones in six healthy females 22-29 years of age. Starting within 6 days after the onset of menses, the subjects ingested a 12-oz portion of soymilk with each of three meals daily for 1 month on a metabolic unit. Daily isoflavone intakes were approximately 100 mg of daidzein (mostly as daidzin) and approximately 100 mg of genistein (mostly as genistin). Serum 17 beta-estradiol levels on cycle days 5-7, 12-14, and 20-22 decreased by 31% (P = 0.09), 81% (P = 0.03), and 49% (P = 0.02), respectively, during soya feeding. Decreases persisted for two or three menstrual cycles after withdrawal from soya feeding. The luteal phase progesterone levels decreased by 35% during soya feeding (P = 0.002). Dehydroepiandrosterone sulfate levels decreased progressively during soya feeding by 14-30% (P = 0.03). Menstrual cycle length was 28.3 +/- 1.9 days before soymilk feeding, increased to 31.8 +/- 5.1 days during the month of soymilk feeding (P = 0.06), remained increased at 32.7 +/- 8.4 days (P = 0.11) at one cycle after termination of soymilk feeding, and returned to pre-soya diet levels five to six cycles later. These results suggest that consumption of soya diets containing phytoestrogens may reduce circulating ovarian steroids and adrenal androgens and increase menstrual cycle length. Such effects may account at least in part for the decreased risk of breast cancer that has been associated with legume consumption.