In recent years, lithium has proved an effective augmentation strategy of antidepressants in both acute and treatment-resistant depression. Neuroprotective and procognitive effects of lithium have been evidenced. Brain-derived neurotrophic factor (BDNF) has been shown to play a key role in the pathophysiology of several neurological and psychiatric disorders. The BDNF hypothesis of depression postulates that a loss of BDNF is directly involved in the pathophysiology of depression, and its restoration may underlie the therapeutic efficacy of antidepressant treatments. Brain-derived neurotrophic factor serum concentrations were measured in a total of 83 acutely depressed patients before and after 4 weeks of lithium augmentation. A significant BDNF increase has been found during treatment (F2,81 = 5.04, P < 0.05). Brain-derived neurotrophic factor concentrations at baseline correlated negatively with relative Hamilton Depression Scale change after treatment with lithium (n = 83; r = -0.23; P < 0.05). This is the first study showing that lithium augmentation of an antidepressant strategy can increase BDNF serum concentrations. Our study replicates previous findings showing that serum BDNF levels in patients with depressive episodes increase during effective antidepressant treatment. Further studies are needed to separate specific effects of different antidepressants on BDNF concentration and address potential BDNF downstream mechanisms.