Several in vitro and in vivo studies have shown that brain-derived neurotrophic factor (BDNF) promotes survival of damaged mesencephalic dopaminergic neurons. Using a specific antibody directed against human recombinant BDNF, we studied the expression of the protein at the cellular level in the post-mortem mesencephalon of control subjects and patients with Parkinson's disease (PD). In control subjects, BDNF was expressed in all mesencephalic regions containing dopaminergic neurons, and in the substantia nigra pars compacta (SNpc) 65% of the melanized neurons expressed BDNF. In the PD SNpc, the total number of pigmented neurons containing BDNF was reduced to 9.6% of the corresponding control value. In contrast, the number of pigmented neurons non-immunoreactive for BDNF was reduced to 23.9% of the corresponding control value. This result appears to indicate that SNpc melanized neurons not expressing BDNF have a 2.5-fold greater probability of surviving than BDNF-positive melanized neurons. Furthermore, we found that in parkinsonian mesencephalon almost all dopaminergic neurons containing Lewy bodies were immunoreactive for BDNF. These findings demonstrate a reduced expression of BDNF in PD and suggest that BDNF protein expression does not protect melanized SNpc neurons from the degenerative process in this disease.