In initially nondepressed patients, lower BDNF levels were not associated with the onset of depression (OR, 1.50; 95% CI, 0.95–2.39; p = 0.081), whereas in patients who were depressed at diagnosis, serum BDNF levels were predictive of depressive symptoms at 6 months (OR, 0.37; 95% CI, 0.19–0.74; p = 0.005). This evidence concerns the gene BDNF and depressive disorder.