Bipolar disorder is a chronic severe mood disorder that has been consistently demonstrated to have a strong inherited component. Traditional approaches to gene discovery have produced conflicting results regarding the association between genes and bipolar disorder. Numerous genes have been proposed as associated with bipolar disorder. This paper will focus on one of these, brain-derived neurotrophic factor (BDNF). BDNF is an interesting candidate gene for bipolar disorder because of its important role in the neurodevelopment of the CNS. Previous genetic work has identified a potential association between a Val66Met polymorphism in the BDNF gene and bipolar disorder. Meta-analysis based on all original published association studies between the Val66Met polymorphism and bipolar disorder up to May 2007 shows modest but statistically significant evidence for the association between the Val66Met polymorphism and bipolar disorder (random-effects pooled odds ratio [OR] = 1.13, 95% Confidence Interval [CI] = 1.04-1.23, Z = 2.85, P = 0.004) from 14 studies consisting of 4248 cases, 7080 control subjects and 858 nuclear families. Further large-scale studies are warranted to elucidate the relevant BDNF gene variation(s) that act as risk factors for bipolar disorder susceptibility.