Brain-derived neurotrophic factor (BDNF) is a neurotrophin that has been linked to the viability of neurons in brain circuits that regulate emotion, memory, learning, sleep, and appetite. BDNF has been most extensively studied in relation to depression. Depressed patients show reduced levels of hippocampal and cortical BDNF in postmortem studies. Recently, to the best of our knowledge, there are at least three meta-analyses regarding blood BDNF levels in depressed patients, suggesting that blood BDNF levels are decreased in depressive state, and those are recovered after treatment with biological treatments such as antidepressants, ECT, and rTMS. From these findings into account, it is possible that blood (plasma and serum) BDNF level is a biological marker for depressive state. We have recently demonstrated that a significantly negative correlation was observed between the HAMD scores and serum BDNF levels. In addition, responders to fluvoxamine, paroxetine, milnacipran, and sertraline all increased serum BDNF levels. Blood BDNF levels did not distinguish between responders and remitters to the treatment. In conclusion, blood BDNF levels partially reflect those in the brain, and there is also strong and consistent evidence indicating that these levels normalize following the biological intervention for depression.