BDNF and major depressive disorder: The three main findings of this study are as follows: (1) SMI patient serum BDNF levels significantly increase during inpatient treatment and increased levels are associated with clinical improvement; (2) estimated mean BDNF levels show no difference among SMI patients with major depression, bipolar depression, manic episode and schizophrenia; and (3) BDNF levels persist at a lower level compared with controls even with the general improvement of clinical scores during the inpatient treatment.