Twenty-seven articles were excluded by reading the full text: 11 studies did not examine BDNF serum levels; 7 and 3 studies did not include patients with CVD and those with depression, respectively; the association between CVD and depression was unclear in 5 studies; and 1 study did not clearly report BDNF serum levels in depressed and nondepressed patients. Here, BDNF is linked to major depressive disorder.