Heterogeneity was assessed using I2 statistics (random-effects model if I2 ≥ 50%; fixed-effects if I2 < 50%).<h4>Results</h4>Serum BDNF levels in patients with depression were significantly lower than those in healthy controls [MD = -1.54, 95% confidence intervals (CI) (-2.85 to -0.24), p = 0.02]. Here, BDNF is linked to depressive disorder.