This study conducted meta-analyses to investigate the relationship between serum brain-derived neurotrophic factor (BDNF) levels and depression, as well as the effects of antidepressant treatment on BDNF levels. The first meta-analysis compared serum BDNF levels between depressed and non-depressed subjects, finding that depressed individuals had significantly lower BDNF levels compared to healthy controls (p < 6.8×10−8). The second meta-analysis compared pre- and post-antidepressant treatment serum BDNF levels, showing significantly higher BDNF levels after treatment (p = 0.003). Both analyses showed no evidence of publication bias and were not unduly influenced by any single study. These findings suggest that serum BDNF levels are abnormally low in patients with major depressive disorder and increase following antidepressant treatment. While the exact mechanisms remain to be determined, these measures may have potential as biomarkers for psychiatric disorders or predictors of antidepressant efficacy.This study conducted meta-analyses to investigate the relationship between serum brain-derived neurotrophic factor (BDNF) levels and depression, as well as the effects of antidepressant treatment on BDNF levels. The first meta-analysis compared serum BDNF levels between depressed and non-depressed subjects, finding that depressed individuals had significantly lower BDNF levels compared to healthy controls (p < 6.8×10−8). The second meta-analysis compared pre- and post-antidepressant treatment serum BDNF levels, showing significantly higher BDNF levels after treatment (p = 0.003). Both analyses showed no evidence of publication bias and were not unduly influenced by any single study. These findings suggest that serum BDNF levels are abnormally low in patients with major depressive disorder and increase following antidepressant treatment. While the exact mechanisms remain to be determined, these measures may have potential as biomarkers for psychiatric disorders or predictors of antidepressant efficacy.