Parkinson disease (PD) is a neurodegenerative disease resulting from the loss of the dopaminergic neurons from the substantia nigra pars compacta (SNpc). It is characterized by bradykinesia, rigidity, resting tremor and/or postural instability. The diagnosis of PD is essentially clinical and there is no reliable biological marker to assess its progression. Recently, investigations have been performed on the potential use of circulating cell-free deoxyribonucleic acid (DNA) in the plasma for clinical diagnosis, prognosis and monitoring of human diseases. The aim of this work was to assess the role of free DNA as a biological marker of PD. Forty-two patients with PD (19 men, 23 women) and 20 healthy (7 men, 13 women) subjects were enrolled in this study. Mean +/- SD plasma DNA concentration in PD patients and control subjects were, respectively, 16,487 +/- 16,378 (range: 100-62,034) kilogenomes-equivalents/L and 37,975 +/- 17,832 (range: 15,143-78,783) kilogenomes-equivalents/L. There was a significant difference between control and PD groups (p < 0.001). There was no correlation between plasma DNA levels and demographic or clinical parameters in PD patients. Free DNA does not seem to be a reliable marker of PD progression. Further research is warranted to confirm the present results to have some value as biomarkers in other neurodegenerative diseases.