The brain-derived neurotrophic factor (BDNF) is a potent inhibitor of apoptosis-mediated cell death and neurotoxin-induced degeneration of dopaminergic neurons. There is a growing body of evidence implicating BDNF in the pathogenesis of Parkinson's disease (PD), suggesting it may eventually be used in the development of neuroprotective therapies for PD. The serum BDNF of 47 PD patients and of 23 control subjects was assessed, and serum BNDF levels were significantly decreased in PD patients when compared with controls (p = 0.046). Interestingly enough, BDNF correlated positively with a longer time span of the disease, as well as with the severity of the PD symptoms and with more advanced stages of the disease. Additionally, higher BDNF levels also correlated with poor balance as assessed by the Berg Balance Scale, more time spent at the Timed Up & Go Test, reduced speed of gait and shorter distance walked during the Six-Minute Walk Test. Our results corroborate the literature regarding the involvement of BDNF in PD. We hypothesize that lower BDNF levels in early stages of the disease may be associated with pathogenic mechanisms of PD. The increase of BDNF levels with the progression of the disease may be a compensatory mechanism in more advanced stages of PD.