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Glycosylation is a cell type-specific post-translational modification that can be used for biomarker identification in various diseases. Aim of this study is to explore glycan-biomarkers on transferrin (Tf) for Alzheimer’s disease (AD) in cerebrospinal fluid (CSF). Glycan structures of CSF Tf were analyzed by ultra-performance liquid chromatography followed by mass spectrometry. We found that a unique mannosylated-glycan is carried by a Tf isoform in CSF (Man-Tf). The cerebral cortex contained Man-Tf as a major isofom, suggesting that CSF Man-Tf is, at least partly, derived from the cortex. Man-Tf levels were analyzed in CSF of patients with neurological diseases. Concentrations of Man-Tf were significantly increased in AD and mild cognitive impairment (MCI) comparing with other neurological diseases, and the levels correlated well with those of phosphorylated-tau (p-tau), a representative AD marker. Consistent with the observation, p-tau and Tf were co-expressed in hippocampal neurons of AD, leading to the notion that a combined p-tau and Man-Tf measure could be a biomarker for AD. Indeed, levels of p-tau x Man-Tf showed high diagnostic accuracy for MCI and AD; 84% sensitivities and 90% specificities for MCI and 94% sensitivities and 89% specificities for AD. Thus Man-Tf could be a new biomarker for AD.