OBJECTIVE: Brain-derived neurotropic factor (BDNF) is widely distributed in the peripheral and central nervous systems. BDNF and its gene polymorphism may be important in synaptic plasticity and neuron survival, and may become a key target in the physiopathology of several mental illnesses. To elucidate the role of BDNF, we compared the plasma BDNF levels and the BDNF Val66Met gene variants effect in several mental disorders. METHOD: We enrolled 644 participants: 177 patients with bipolar I disorder (BP-I), 190 with bipolar II disorder (BP-II), 151 with schizophrenia, and 126 healthy controls. Their plasma BDNF levels and BDNF Val66Met single nucleotide polymorphisms (SNP) were checked before pharmacological treatment. RESULTS: Plasma levels of BDNF were significantly lower in patients with schizophrenia than in healthy controls and patients with bipolar disorder (F = 37.667, p<0.001); the distribution of the BDNF Val66Met SNP was not different between groups (χ(2) = 5.289, p = 0.507). Nor were plasma BDNF levels significantly different between Met/Met, Met/Val, and Val/Val carriers in each group, which indicated that the BDNF Val66Met SNP did not influence plasma BDNF levels in our participants. Plasma BDNF levels were, however, significantly negatively correlated with depression scores in patients with bipolar disorder and with negative symptoms in patients with schizophrenia. CONCLUSION: We conclude that plasma BDNF profiles in different mental disorders are not affected by BDNF Val66Met gene variants, but by the process and progression of the illness itself.