BACKGROUND: Although preclinical studies suggest that fermented soy foods may have a protective effect against breast cancer, no prospective cohort studies have examined this association. OBJECTIVE: Our study examined the association between fermented and nonfermented soy food intake and breast cancer risk using a population-based prospective cohort study in Japan. METHODS: We included a total of 47,614 women aged 45-74 years in an analysis of the Japan Public Health Center-based Prospective Study (JPHC Study). A validated food frequency questionnaire (FFQ) was used for the assessment of dietary intake. Breast cancer incidence was analyzed by multivariate Cox proportional hazards regression models. RESULTS: During an average of 15.5 years of follow-up, 825 breast cancer cases were newly identified. We found no association of intake of soy foods with breast cancer risk, regardless of fermentation, with multivariate hazard ratios (HRs) and 95% confidence intervals (CIs) for the highest quartiles of fermented and nonfermented soy food intake of 0.94 (0.67, 1.32) and 1.15 (0.85, 1.57) compared with the lowest quartile (p for trend = 0.305 and 0.393). Unlike nonfermented soy, higher intake of fermented soy foods was associated with a significant decrease in the risk of nonlocalized breast cancer. The HR and 95% CI in the highest compared to lowest intake category of fermented soy foods was 0.53 (0.28, 0.99) versus nonfermented soy foods 0.85 (0.51, 1.42) (p for trend = 0.026 and 0.797). CONCLUSIONS: Our analyses showed that fermented soy foods had no association with overall breast cancer but may be associated with decreased risk of nonlocalized breast cancer.