OBJECTIVES To test the hypothesis that low circulating brain-derived neurotrophic factor (BDNF), a secretory member of the neurotrophin family that has a protective role in neurodegeneration and stress responses and a regulatory role in metabolism, predicts risk of all-cause mortality in 85-year-old men and women. DESIGN Longitudinal study with 50- to 58-month follow-up. SETTING The 1914 cohort, a population-based cohort established in 1964 by the Research Center for Prevention and Health at Glostrup Hospital. PARTICIPANTS One hundred eighty-eight unselected 85-year-old Danes. MEASUREMENTS BDNF was measured in plasma and serum. The Danish National Register of Patients was used to collect data on morbidity. The primary outcome in Cox regression analyses was all-cause mortality. RESULTS Women with low plasma BDNF (lowest tertile) had greater all-cause mortality risk than women with high plasma BDNF (highest tertile) (hazard ratio=2.2, 95% confidence interval=1.1-4.7). Low plasma BDNF predicted mortality independently of activities of daily living; education; and a history of central nervous system disease, cerebrovascular accidents, cardiovascular disease, cancer, respiratory disease, and low-grade inflammation. No association was found between plasma BDNF and mortality in men, and serum BDNF did not influence mortality in either sex. CONCLUSION Low plasma BDNF is a novel, independent, and robust biomarker of mortality risk in old women. BDNF may be a central factor in the network of multimorbidity in old populations.