Context: Folate is involved in DNA synthesis and methylation and may reduce breast cancer risk, particularly among women with greater alcohol consumption. Objectives: To assess the association between folate intake and risk of breast cancer and whether higher folate intake may reduce excess risk among women who consume alcohol. Design: Prospective cohort study performed in 1980, with 16 years of follow-up. Setting and Participants: A total of 88 818 women who completed the dietary questionnaire section of the Nurses' Health Study in 1980. Main Outcome Measure: Incidence of invasive breast cancer by levels of folate and alcohol intake. Results: A total of 3483 cases of breast cancer were documented. Total folate intake was not associated with overall risk of breast cancer. However, among women who consumed at least 15 g/d of alcohol, the risk of breast cancer was highest among those with low folate intake. For total folate intake of at least 600 μg/d compared with 150 to 299 μg/d, the multivariate relative risk (RR) was 0.55 (95% confidence interval [CI], 0.39-0.76; P for trend = .001). This association was only slightly attenuated after additional adjustment for intake of beta carotene, lutein/zeaxanthin, preformed vitamin A, and total vitamins C and E. The risk of breast cancer associated with alcohol intake was strongest among women with total folate intake of less than 300 μg/d (for alcohol intake ≥15 g/d vs <15 g/d, multivariate RR, 1.32; 95% CI, 1.15-1.50). For women who consumed at least 300 μg/d of total folate, the multivariate RR for intake of at least 15 g/d of alcohol vs less than 15 g/d was 1.05 (95% CI, 0.92- 1.20). Current use of multivitamin supplements, the major source of folate, was associated with lower breast cancer risk among women who consumed at least 15 g/d of alcohol (for current users of supplements vs never users, RR, 0.74; 95% CI, 0.59-0.93). Conclusions: Our findings suggest that the excess risk of breast cancer associated with alcohol consumption may be reduced by adequate folate intake.