Accumulating evidence suggest that neural changes and cognitive impairment may accompany the course of bipolar disorder. Such detrimental effects of cumulative mood episodes may be related to changes in neurotrophins that take place during mood episodes but not during euthymic phases. The present study investigated serum neurotrophin-3 (NT-3) levels in patients with bipolar disorder during manic, depressed, and euthymic states, using an enzyme-linked immunosorbent assay (sandwich-ELISA). Serum NT-3 levels were increased in manic (p<0.001) and depressed (p<0.001) BD patients, as compared with euthymic patients and normal controls. These findings suggest that the NT-3 signaling system may play a role in the pathophysiology of BD.