Folate's role in breast cancer development is controversial. Not only estrogen receptor (ER) a status, but also ER3 status of tumors may have confounded results from previous epidemiological studies. We aimed to examine associations between plasma folate concentration and postmenopausal breast cancer defined by ER status. This nested case-control study, within the Malmo diet and cancer cohort, included 204 incident breast cancer cases with information on ER[alpha] and ER[beta] status determined by immunochemistry on tissue micro-array sections. Plasma folate concentration was analyzed for the cases and 408 controls (matched on age and blood sample date). Odds ratios (OR) for ER-defined breast cancers in tertiles of plasma folate concentration were calculated with unconditional logistic regression. All tests were 2-sided. Women in the third tertile of plasma folate concentration ( 12 nmol/L) had higher incidence of ER[beta]--breast cancer than women in the first tertile (OR: 2.67; 95% CI: 1.44-4.92; P-trend = 0.001). We did not observe significant associations between plasma folate concentration and other breast cancer subgroups defined by ER status. We observed a difference between risks for ER[beta]+ and ER[beta]- cancer (P-heterogeneity = 0.003). Our findings, which indicate a positive association between plasma folate and ER[beta]- breast cancer, highlight the importance of taking ER[beta] status into consideration in studies of folate and breast cancer. The study contributes knowledge concerning folate's multifaceted role in cancer development. If replicated in other populations, the observations may have implications for public health, particularly regarding folic acid fortification. J. Nutr. 140: 1661-1668, 2010. doi: 10.3945/jn.110.124313.