BDNF and Alzheimer disease: MCI group had significantly lower serum BDNF levels compared to the control group. AD group had a downward trend of serum BDNF levels than control group, but was not statistically significant. BDNF levels have a positive correlation with Aβ42 levels in CSF. The decreased serum BDNF levels could potentially be used as an AD early detection and progression biomarker.