Soy intakes have been associated with reduced rates of breast cancer in some Asian populations. The isoflavones daidzein and genistein and other components of soybeans may modulate endocrine function and lead to beneficial health effects. This study determined the effects of a soy diet containing minimum amounts of isoflavones on circulating levels of ovarian hormones and gonadotropins. Nine healthy, regularly cycling women consumed a constant soya-contain- ing diet on a metabolic unit starting on day 2 of a menstrual cycle until day 2 of the next cycle. The soy diet was calculated to maintain constant body weight and included a 36-oz portion of soymilk that provided 334 kilocalories and less than 5 mg/day of total isoflavones. The energy distribution of the soy diet was 35.9% fat, 14.0% protein, and 49.8% carbohydrate whereas the home diets averaged 39% fat, 16.6% protein, and 42.5% carbohydrate. For the group, the soya diet provided more carbohydrate (P 5 0.002) and less protein (P 5 0.005) than the home diets. Daily consumption of the soya diet reduced daily circulating levels of 17b-estradiol over the entire menstrual cycle by 20% (P , 0.01, paired t test, two-tailed) and progesterone by 33% (P , 0.0001) compared with levels during the home diet period, but had no effect on LH, FSH, or sex hormone-binding globulin. The decreases in follicular phase 17b-estradiol during the soy diet can be accounted for by changes in energy intakes, nutrient density, and fiber intake, whereas changes in luteal phase 17b-estradiol were most strongly associated with differences in fiber intake. Changes in progesterone levels were most strongly associated with changes in protein intake and much less with other nutrients. Isoflavones were not detectable in plasma and urine during either the soy or home diet periods. These results suggest that at least under the conditions of this study, a soy diet with low levels of isoflavones and low energy intake from protein can reduce circulating ovarian steroids without altering gonadotro- pins. Our results are consistent with previous studies showing de- creased ovarian hormone levels and decreased risk of breast cancer in populations consuming soya diets and an inverse relationship be- tween animal protein intake and breast cancer risk and, therefore, may have implications for breast cancer prevention. (J Clin Endo- crinol Metab 86: 3045-3052, 2001)