The aim of the present study was to compare serum brain-derived neurotrophic factor (BDNF) levels of patients with major depressive disorder (MDD) and conversion disorder (CD). Serum BDNF levels were measured in the following three groups: 15 CD patients without any comorbid diagnosis of psychiatric disorder, 24 patients with MDD, and 26 healthy subjects without any psychiatric diagnosis or psychiatric treatment. The serum BDNF level of the healthy control group (31.4 +/- 8.8 ng/mL) was statistically higher than the level of the MDD group (21.2 +/- 11.3 ng/mL) and the CD group (24.3 +/- 9.0 ng/mL; P = 0.008). This suggests that BDNF level may play a similar role in the pathophysiology of MDD and CD.