We examined the cross-sectional relationship of soy product intake to serum concentrations of estradiol (E2) and sex hormone-binding globulin (SHBG) in 50 healthy premenopausal Japanese women. Intakes of soy products and various nutrients were estimated through a semiquantitative food-frequency questionnaire. Blood samples were drawn from each subject on Days 11 and 22 of her menstrual cycle. The intake of soy products was inversely correlated with E2 on Days 11 and 22 of the cycle after controlling for age, body mass index, cycle length, and intakes of total energy, fat, and crude fiber (r = -0.23, p = 0.08 and r = -0.32, p = 0.04, respectively). No significant correlation was observed between soy product intake and SHBG. There was a tendency for the intake of individual soy foods such as tofu and miso to be inversely correlated with E2 on Days 11 and 22 of the menstrual cycle. Miso intake was inversely significantly correlated with SHBG on Day 22 of the cycle (r = -0.36, p = 0.02). If the observed association is causal, our results suggest that the consumption of soy products lowers the risk of developing breast cancer risk modifying estrogen metabolism.