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Isoflavones, phytoestrogens contained in soy foods, may play a role in breast cancer prevention. This randomized double-blinded trial with 34 premenopausal women investigated whether 100 mg of isoflavones per day versus placebo affects the ovulatory cycle during 1 year. Compliance with the study regimen was confirmed by the increase of urinary isoflavone excretion among the intervention group. Blood samples were taken 5 days after ovulation as determined by an ovulation kit, at baseline, and at months 1, 3, 6, and 12. Serum levels of estrone, estradiol, estrone sulfate, progesterone, sex hormone-binding globulin, follicle-stimulating hormone, and luteinizing hormone were quantified by immunoassay; free estradiol was calculated. We applied the method of least squares to fit general linear models to test for an intervention effect while taking into account the repeated measurement design. Except for a …