Although not uniformly consistent, epidemiologic studies generally suggest an inverse association between dietary intake and blood measurements of folate and breast cancer risk. However, the Prostate, Lung, Colorectal, and Ovarian (PLCO) Cancer Screening trial has recently reported for the first time a potential harmful effect of high folate intake on breast cancer risk. In this study, the risk of developing breast cancer was significantly increased by 20% in women reporting supplemental folic acid intake > or = 400 microg/d compared with those reporting no supplemental intake. Furthermore, although food folate intake was not significantly related to breast cancer risk, total folate intake, mainly from folic acid supplementation, significantly increased breast cancer risk by 32%. The data from the PLCO trial support prior observations made in epidemiologic, clinical, and animal studies suggesting that folate possesses dual modulatory effects on the development and progression of cancer depending on the timing and dose of folate intervention. Based on the lack of compelling supportive evidence, routine folic acid supplementation should not be recommended as a chemopreventive measure against breast cancer at present.