Background: The pathogenesis of substantia nigra pars compacta neuronal injury in Parkinson disease (PD) remains unknown. Cerebrospinal fluid (CSF) has been reported to contain factors toxic to dopaminergic neurons. Objectives: To determine whether the cytotoxic effects of CSF of PD patients are specific for dopaminergic neurons, dependent on prior levodopa therapy, and mediated by the cytokine tumor necrosis factor α (TNF-α). Design: Specimens of CSF were evaluated in dopaminergic (MES 23.5) and nondopaminergic (N18TG2) cell lines for cytotoxicity by viability assay and by the inhibition of tyrosine hydroxylase. After specificity and time and dose response were established, CSF specimens were assayed in a blinded manner. The TNF-α levels in CSF were determined by enzyme-linked immunosorbent assay. The toxicity of TNF-α in MES 23.5 cells was determined. Setting: A university-based research facility. Subjects: There were 4 groups of subjects: normal control subjects (n = 10), control subjects with neurologic disease (n = 8), PD patients treated with levodopa (n = 10), and untreated subjects with PD (n = 20). Results: Specimens of CSF from 15 (50%) of 30 PD patients and 2 (11%) of 18 control subjects were cytotoxic to dopaminergic MES 23.5 cells and were nontoxic to the parental cell line N18TG2. There was no correlation between the degree of PD CSF cytotoxicity, levodopa therapy or the severity and duration of PD. Terminal deoxynucleotidyl transferase-mediated biotin-deoxy-uridine triphosphate nick- end labeling (TUNEL) for DNA fragmentation suggested the involvement of apoptotic mechanisms. The inhibition of tyrosine hydroxylase was an early effect of cell injury by PD CSF and correlated with the viability assay. The mean TNF-α level was 2.6-fold higher in CSF specimens from PD patients than in those of controls. The addition of recombinant human TNF-α equivalent to the highest level determined in PD CSF was not cytotoxic to MES 23.5 cultures. Conclusions: Blinded CSF specimens from PD patients regardless of therapy, contain factors that cause specific dopaminergic neuronal cell injury. These factors are present in a substantial proportion of CSF specimens from patients with early PD, before the institution of medical therapy. Levels of TNF-α are elevated in the CSF of PD patients, but TNF-α is not responsible for the cytotoxicity.