This study tried to investigate the relationships between serum brain-derived neurotrophic factor (BDNF) protein levels and major depressive patients and discuss the effects of antidepressants on the serum BDNF protein levels. A total of 218 participants, including 111 patients with major depression (91 women) and 107 healthy controls (65 women), were recruited in this study. Serum BDNF protein levels were measured using an ELISA kit. Psychiatric diagnoses were made according to DSM-IV criteria. Severity of major depression was assessed by the 17-item Hamilton Depression Rating Scale. Using analysis of covariance with age adjustment, there were significantly low serum BDNF protein levels in depressive patients than healthy controls in women (F=7.530, p=0.007), but not in men. Additionally, changes in serum BDNF protein levels were significantly increased in 79 patients taking antidepressants during a period of 4 weeks (t=2.116, p=0.038), especially in 61 women (t=2.542, p=0.014). Age-adjusted ANCOVA revealed no significant differences in serum BDNF protein levels between 58 responders and 21 non-responders (F=0.008, P=0.928). In responders, there were significantly increased changes in serum BDNF protein levels in 44 women (t=2.501, p=0.016), but not in 14 men (t=-0.767, p=0.457). These analytical results suggest that low serum BDNF may play an important role in depressive women and antidepressant treatment significantly increase serum BDNF. However, further studies of larger populations are necessary to confirm these results and further elucidate the effects of different classes of antidepressants on serum BDNF protein levels.