Overall, random-effects between-group meta-analysis showed that peripheral BDNF levels were decreased in subjects with BD in mania with moderate effect sizes (g = −0.57, 95 % CI −0.99 to −0.14, P = 0.010, 19 between-group comparisons, n = 1,397) and decreased in depression with large effect sizes (g = −0.93, 95 % CI −1.37 to −0.50, P = 0.001, 15 between-group comparisons, n = 1,074) when compared to healthy controls. This evidence concerns the gene BDNF and major depressive disorder.